arm64: efi: only attempt efi map setup if booting via EFI

Booting a kernel with CONFIG_EFI enabled on a non-EFI system caused
an oops with the current UEFI support code.
Add the required test to prevent this.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>