sh-pfc: Add support for SoC-specific initialization

Add two optional init and exit SoC operations and call them from the
core at probe and remove time.

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>