On arm64 the its we use to generate the test FIT image has
arch = "arm";
We should use "arm64" here which is mapped to IH_ARCH_ARM64 via
uimage_arch[].
Fixes: 8391f95549 ("test/py: Create a test for launching UEFI binaries from FIT images")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>