arm64: kconfig: allow CPU_BIG_ENDIAN to be selected

This patch wires up CONFIG_CPU_BIG_ENDIAN for the AArch64 kernel
configuration.

Selecting this option builds a big-endian kernel which can boot into a
big-endian userspace.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -139,6 +139,11 @@ config ARM64_64K_PAGES
 	  look-up. AArch32 emulation is not available when this feature
 	  is enabled.
 
+config CPU_BIG_ENDIAN
+       bool "Build big-endian kernel"
+       help
+         Say Y if you plan on running a kernel in big-endian mode.
+
 config SMP
 	bool "Symmetric Multi-Processing"
 	select USE_GENERIC_SMP_HELPERS