sh: Add gpio.h stubs for PFC definitions.

This adds the PFC definitions for SH-3, as well as consolidating the
gpio.h mess within sh-sci. Stub in sh64, as it's the odd one out
between the sh-sci architectures (sh, sh64, h8300) in this capacity.

Signed-off by: Markus Brunner <super.firetwister@gmail.com>
Signed-off by: Mark Jonas <toertel@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>