sh: pfc: support pinmux deregistration.

Presently the pinmux code is a one-way thing, but there's nothing
preventing an unregistration if no one has grabbed any of the pins.
This will permit us to save a bit of memory on systems that require pin
demux for certain peripherals in the case where registration of those
peripherals fails, or they are otherwise not attached to the system.

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