efi/x86: earlyprintk - Add 64bit efi fb address support

EFI GOP uses 64-bit frame buffer address in some BIOS.
Add 64bit address support in efi earlyprintk.

Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>