soc: ti: mspm0: restructure common files

mspm0 series (currently g, l and c) shares the common SoC
level init functions and pin control/muxing configurations.

To avoid duplication for each series, create a common path
and move the SoC and pin control definitions.

Other common functionalities like Power Management, Power off
handling will be added in future, which will be common across
series.

Signed-off-by: Parthiban Nallathambi <parthiban@linumiz.com>