uboot: arm64 arch build support

For a 64bit arm architecture, Buildroot uses arm64 terminology and
from the top-level Makefile KERNEL_ARCH is set to arm64 which is then
passed to the uboot build.  This causes a compilation issue as uboot
uses the top-level system architecture as it's $(ARCH).  So
arch/$(ARCH)/Makefile doesn't work with arm64 unless we adjust the
arch.

[Thomas:
 - rewrap commit message text.
 - simplify comment in the code.]

Signed-off-by: Ronak Desai <ronak.desai@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>