xen/arm: mpu: Ensure that the page size is 4KB

Similar to commit (d736b6eb45, "xen/arm: mpu: Define Xen start address for
MPU systems"), one needs to add a build assertion to ensure that the page size
is 4KB on arm32 based systems as well.
The existing build assertion is moved under "xen/arch/arm/mpu" as it applies
for both arm64 and arm32 based systems.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Acked-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>

xen/arch/arm/Makefile

@@ -1,6 +1,7 @@
 obj-$(CONFIG_ARM_32) += arm32/
 obj-$(CONFIG_ARM_64) += arm64/
 obj-$(CONFIG_MMU) += mmu/
+obj-$(CONFIG_MPU) += mpu/
 obj-$(CONFIG_ACPI) += acpi/
 obj-$(CONFIG_HAS_PCI) += pci/
 ifneq ($(CONFIG_NO_PLAT),y)

xen/arch/arm/arm64/mpu/Makefile

@@ -1,2 +1 @@
 obj-y += head.o
-obj-y += mm.o

xen/arch/arm/mpu/Makefile

@@ -0,0 +1 @@
+obj-y += mm.o