Add Cortex-A53 port with system register interface for CPU interface access (#357)

The difference between this port and portable/GCC/ARM_CA53_64_BIT is
that this port uses System Register interface to access CPU interface
while the other one uses Memory-mapped interface.

Signed-off-by: Gaurav Aggarwal 
Co-authored-by: Cobus van Eeden <[email protected]>

Author: aggarg