barebox: fix ARCH value for arm64

barebox 2016.08 added arm64 support using ARCH=arm.

[Peter: extend commit message to clarify]
Signed-off-by: Raphael Poggi <poggi.raph@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/barebox/barebox.mk

@@ -61,6 +61,8 @@ else ifeq ($$(KERNEL_ARCH),x86_64)
 $(1)_ARCH = x86
 else ifeq ($$(KERNEL_ARCH),powerpc)
 $(1)_ARCH = ppc
+else ifeq ($$(KERNEL_ARCH),arm64)
+$(1)_ARCH = arm
 else
 $(1)_ARCH = $$(KERNEL_ARCH)
 endif