barebox: set ARCH=arm for aarch64
barebox: set ARCH=arm for aarch64

Barebox doesn't handle ARM-32Bit and ARM-64bit separately. So we can not use
ARCH=arm64 for barebox. We have to set it to ARCH=arm.

Signed-off-by: Christian Hemp <e88dc360e19917db49e9381505677127ff56266d@phytec.de>
Signed-off-by: Stefan Müller-Klieser <a3e08e35e4a42be768e7d0e76ae1ba321c2a66dd@phytec.de>

PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec