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>

arch/arm/Kconfig-nommu

@@ -53,6 +53,7 @@ config REMAP_VECTORS_TO_RAM
 
 config ARM_MPU
        bool 'Use the ARM v7 PMSA Compliant MPU'
+       depends on CPU_V7
        default y
        help
          Some ARM systems without an MMU have instead a Memory Protection