sh: pfc: Split out gpio chip support.

This implements a bit of rework for the PFC code, making the core itself
slightly more pluggable and moving out the gpio chip handling completely.

The API is preserved in such a way that platforms that depend on it for
early configuration are still able to do so, while making it possible to
migrate to alternate interfaces going forward.

This is the first step of chainsawing necessary to support the pinctrl
API, with the eventual goal being able to decouple pin function state
from the gpio API while retaining gpio chip tie-in for gpio pin functions
only, relying on the pinctrl/pinmux API for non-gpio function demux.

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