ARM: mpu: Ensure that MPU depends on CPU_V7

The support for the MPU is currently implemented only for R-class
(PMSAv7/R). Since the merge of V7M support in to the kernel it is possible
to select MPU support on V7M.

This patch ensures that until MPU support for M-class processors is
implemented, the MPU can only be selected with R-class CPUs

Signed-off-by: Jonathan Austin <jonathan.austin@arm.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>