[feature-nrf528xx] NRF52 SPI and I2C re-implementation

[marcuschangarm]

This PR re-implements SPI and I2C in a way that lets both drivers share the underlying hardware. From the new README.md file:

SPI and I2C

The TWI/TWIM (I2C) and SPI/SPIM module shares the same underlying hardware and each instance can only provide one functionality at a time. Both the NRF52832 and NRF52840 have 2 TWI/TWIM modules and 3 SPI/SPIM:

Instance 0	Instance 1	Instance 2
SPI0/SPIM0	SPI1/SPIM1	SPI2/SPIM2
TWI0/TWIM0	TWI1/TWIM1

When instantiating a new Mbed SPI object or I2C object, the object will be assigned a hardware instance. By default, the HAL implementation will automatically pick a hardware instance based on the assigned pins in the following order:

1. The driver will look up the pins in the configuration table and pick a pre-assigned instance.
2. If the pins can't be found in the configuration table, the driver will check if a hardware instance has already been assigned to those pins so that objects using the same pins will share the same instance.
3. If no instance has been assigned, the driver will look for a free instane. For I2C objects instances are assigned from 0 to 1. For SPI objects instances are assigned from 2 to 0. This ensures that no matter the order of instantiation the first three objects can be guaranteed to be on separate instances no matter the pins being used.
4. If no unused instance can be found, objects not sharing any pins will be assigned to the same default instance, which is Instance 0 for I2C and Instance 2 for SPI.

Customization

A custom configuration table can be provided by overriding the weakly defined default empty table. In the example below, I2C objects using pins p1 and p2 for SDA and SCL will always be assigned to Instance 1 and SPI objects using pins p3, p4, p5 for MOSI, MISO, and CLOCK will be assigned to Instance 2 and SPI objects using pins p6, p7, and p8 will be assigned to Instance 0. The custom configuration table must always be terminated with a row of NC.

const PinMapI2C PinMap_I2C[] = { 
    {p1, p2, 1},
    {NC, NC, NC}
};

const PinMapSPI PinMap_SPI[] = {
    {p3, p4, p5, 2},
    {p6, p7, p8, 0},
    {NC, NC, NC, NC}
};

The tables must be placed in a C compilation file.

Concurrency

1. When called from the same thread, it is safe to assign I2C and SPI objects to the same instance.
2. If an instance is being used exclusively for either I2C or SPI, the objects can safely be called from multiple threads.
3. If an instance is being used for both I2C and SPI, the user must provide thread safety between the objects.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1420
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6296/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[cmonr]

Test CI canceled due to 5.8.0-RC2 generation. Will resume once 5.8.0-RC2 is released.

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1068
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6296/

[cmonr]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 1215
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6296/1215