arm64: Kconfig: select COMPAT_BINFMT_ELF only when BINFMT_ELF is set

Fix warning:
"(COMPAT) selects COMPAT_BINFMT_ELF which has unmet direct dependencies
(COMPAT && BINFMT_ELF)"

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>