sh: Add PFC platform device registration helper function

The sh_pfc_register() function can be called by boards or SoC setup code
to register the PFC platform device.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>