arm64: efi: add efi-entry.o to targets instead of extra-$(CONFIG_EFI)

efi-entry.o is built on demand for efi-entry.stub.o, so you do not have
to repeat $(CONFIG_EFI) here. Adding it to 'targets' is enough.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>