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>

arch/arm64/kernel/efi.c

@@ -333,6 +333,9 @@ void __init efi_init(void)
 
 void __init efi_idmap_init(void)
 {
+	if (!efi_enabled(EFI_BOOT))
+		return;
+
 	/* boot time idmap_pg_dir is incomplete, so fill in missing parts */
 	efi_setup_idmap();
 }