sh: pfc: Rudimentary pinctrl-backed GPIO support.

This begins the migration of the PFC core to the pinctrl subsystem.
Initial support is very basic, with the bulk of the implementation simply
being nopped out in such a way to allow registration with the pinctrl
core to succeed.

The gpio chip driver is stripped down considerably now relying purely on
pinctrl API calls to manage the bulk of its operations.

This provides a basis for further PFC refactoring, including decoupling
pin functions from the GPIO API, establishing pin groups, and so forth.
These will all be dealt with incrementally so as to introduce as few
growing and migratory pains to tree-wide PFC pinmux users today.

When the interfaces have been well established and in-tree users have
been migrated off of the legacy interfaces it will be possible to strip
down the core considerably, leading to eventual drivers/pinctrl rehoming.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>