sh: improve pinmux support for single direction pins

This patch improves the support for gpio pins that are hard wired
to either input or output and lack control register association.
A special force enum id is used to allow use without control
register but still mark the gpio pin as input or output.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>