arch: aarch64 always has a MMU

Following the addition of AArch64 big endian, the AArch64 little
endian option had lost its 'select BR2_ARCH_HAS_MMU_MANDATORY', so
let's reintroduce it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

arch/Config.in

@@ -61,6 +61,7 @@ config BR2_armeb
 config BR2_aarch64
 	bool "AArch64 (little endian)"
 	select BR2_ARCH_IS_64
+	select BR2_ARCH_HAS_MMU_MANDATORY
 	help
 	  Aarch64 is a 64-bit architecture developed by ARM Holdings.
 	  http://www.arm.com/products/processors/instruction-set-architectures/armv8-architecture.php