efi: Rename UCLASS_EFI and IF_TYPE_EFI

These names are better used for access to devices provided by an EFI
layer. Use EFI_LOADER instead here, since these are only available in
U-Boot's EFI_LOADER layer.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>