arm/mpu: Add HYPERVISOR_VIRT_START and avoid a check in xen.lds.S

The define HYPERVISOR_VIRT_START is required by the common code,
even if MPU system doesn't use virtual memory, define it in
mpu/layout.h in order to reuse existing code.

Disable a check in the linker script for arm for !MMU systems.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>

xen/arch/arm/include/asm/mpu/layout.h

@@ -22,6 +22,8 @@
  */
 #define XEN_VIRT_START         _AT(paddr_t, XEN_START_ADDRESS)
 
+#define HYPERVISOR_VIRT_START  XEN_VIRT_START
+
 #endif /* __ARM_MPU_LAYOUT_H__ */
 /*
  * Local variables:

xen/arch/arm/xen.lds.S

@@ -237,4 +237,6 @@ ASSERT(IS_ALIGNED(__bss_start,      POINTER_ALIGN), "__bss_start is misaligned")
 ASSERT(IS_ALIGNED(__bss_end,        POINTER_ALIGN), "__bss_end is misaligned")
 /* To simplify the logic in head.S, we want to _end to be page aligned */
 ASSERT(IS_ALIGNED(_end,             PAGE_SIZE), "_end is not page aligned")
+#ifdef CONFIG_MMU
 ASSERT((_end - _start) <= XEN_VIRT_SIZE, "Xen is too big")
+#endif