libpayload: Provide API to expose MMU memery ranges for ARM64

Provide get_mmu_ranges() for ARM64 to let payloads could get
MMU ranges for all used memory regions.

BUG=b:171858277
TEST=Build in x86, arm, arm64.
  emerge-zork libpayload depthcharge
  emerge-nyan libpayload depthcharge
  emerge-asurada libpayload depthcharge

Signed-off-by: Meng-Huan Yu <menghuan@google.com>
Change-Id: I39b24aefc9dbe530169b272e839d0e1e7c697742
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48113
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>

payloads/libpayload/arch/arm64/mmu.c

@@ -705,3 +705,8 @@ void mmu_presysinfo_enable(void)
 	mmu_init(&usedmem_ranges);
 	mmu_enable();
 }
+
+const struct mmu_ranges *mmu_get_used_ranges(void)
+{
+	return &usedmem_ranges;
+}