soc: imxrt7xx: Split Kconfig files to CPU folders

For better organization, split the Kconfig files into one per each CPU.

Also, there was a bug where MFD was made to depend on flexcomm being
enabled, when really it probably meant to just default y if flexcomm is
enabled.

Leave Kconfig.soc in one file for the SOC.

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