Add a generic way to describe bias registers (for pull-up/down control),
like is already done for config and drive registers.

This makes the sh-pfc core code aware of these registers, which will
ease introducing suspend/resume support later.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>