efi/x86: move UV_SYSTAB handling into arch/x86

The SGI UV UEFI machines are tightly coupled to the x86 architecture
so there is no need to keep any awareness of its existence in the
generic EFI layer, especially since we already have the infrastructure
to handle arch-specific configuration tables, and were even already
using it to some extent.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>