soc: nxp: mcx: Fix Kconfig files and MFD config

The Kconfig defaults should be in Kconfig.defconfig.

MFD should not be made dependent on the LP flexcomm being
enabled, since MFD can be for other things than just flexcomm.
Change from depends on to an if.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>