efi/arm64: Handle missing virtual mapping for UEFI System Table

If we cannot resolve the virtual address of the UEFI System Table, its
physical offset must be missing from the virtual memory map, and there
is really no point in proceeding with installing the virtual memory map
and the runtime services dispatch table. So back out gracefully.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Mark Salter <msalter@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>