scripts/mkknlimg assumes ARCH_BCM283x when bcm2835_wdt is builtin

[notro]

@pelwell Can we drop this:

        $is_283x ||= config_bool($res, 'brcm,bcm2835-pm-wdt');

I want to have this driver builtin on ARCH_BCM270x to take advantage of it's restart/poweroff code, but currently that means I have to use --dtok to get the right DT.

[pelwell]

Good call. Done.