rootfs_deb: handle aarch64 SDK_ARCH

This class maps from OE architectures to dpkg architectures, but wasn't
handling aarch64 correctly.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 315fe8ab)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>