soc: arm: nxp: kinetis: k2x: remove MPU support

Remove (disabled) MPU support for the NXP Kinetis K2x. At least the
MK22F51212, which is the only K2x supported by Zephyr so far, does not
contain an MPU.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>