pinctrl: sh-pfc: r8a7796: Add bias pinconf support

Implements pull-up and pull-down. On this SoC there is no simple mapping
of GP pins to bias register bits, so we need a table.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>