lib: arm/arm64: Refuse to disable the MMU with non-identity stack pointer

When the MMU is off, all addresses are physical addresses. If the stack
pointer is not an identity mapped address (the virtual address is not the
same as the physical address), then we end up trying to access an invalid
memory region. This can happen if we call mmu_disable from a secondary CPU,
which has its stack allocated from the vmalloc region.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>