xen/arm: Introduce CONFIG_MMU Kconfig option

There are two types of memory system architectures available for
Arm-based systems, namely the Virtual Memory System Architecture (VMSA)
and the Protected Memory System Architecture (PMSA). According to
ARM DDI 0487G.a, A VMSA provides a Memory Management Unit (MMU) that
controls address translation, access permissions, and memory attribute
determination and checking, for memory accesses made by the PE. And
refer to ARM DDI 0600A.c, the PMSA supports a unified memory protection
scheme where an Memory Protection Unit (MPU) manages instruction and
data access. Currently, Xen only supports VMSA.

Introduce a Kconfig option CONFIG_MMU, which is currently default
set to y and unselectable because currently only VMSA is supported.
CONFIG_MMU will be used in follow-up patches.

Suggested-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Henry Wang <Henry.Wang@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>

xen/arch/arm/Kconfig

@@ -59,6 +59,9 @@ config PADDR_BITS
 	default 40 if ARM_PA_BITS_40
 	default 48 if ARM_64
 
+config MMU
+	def_bool y
+
 source "arch/Kconfig"
 
 config ACPI