nrf51 - document the legacy mcu support vs the new one?

[0xc0170]

There are currently two nrf5x HAL, and it's not documented anywhere. For users to use or bugfix, we should provide a documentation reference to explain.

I assume the old one is "legacy" and might be removed once all the rest is ported to the new one.

Folder reference: https://github.com/mbedmicro/mbed/tree/master/hal/targets/cmsis/TARGET_NORDIC/

cc @pan-

[pan-]

@0xc0170 You are correct, there should be a document explaining how to transition from the old HAL to the new one.

I don't expect users to be impacted by those changes (maybe I miss cases but please enlighten me), I also see bugfixes coming to both HAL but for porters this is a critical document to provide.

CC @nvlsianpu @anangl

[ciarmcom]

ARM Internal Ref: IOTMORF-102