arch: beetle: Remove ARM_MPU_ENABLE option.

The ARM_MPU_ENABLE option is just selecting ARM_MPU option,
which could be also controlled through menuconfig.

This commit removes the ARM_MPU_ENABLE option and replaces
its usage by ARM_MPU option.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>