ARM: mpu: Allow enabling of the MPU via kconfig

Allows the user to select MPU support when compiling for ARM processors
that support the PMSAv7.

This ensures that CONFIG_SMP depends on the MPU in the case that no MMU
is present.

CONFIG_SMP_ON_UP is not implemented for nommu, so introduce an MMU
dependency there.

Signed-off-by: Jonathan Austin <jonathan.austin@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>