ext: hal: ti: msp432p4xx: Tweak SDK to build with Zephyr

There are some minor conflicts or porting changes needed to get the
msp432p4xx SDK to work with Zephyr:
1. Remove definition of BIT, as it conflicts with Zephyr defined one
2. Set __SYSTEM_CLOCK define based on Zephyr Kconfig define
3. Fix a build warning that gets treated as an error.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>