pinctrl: renesas: Factor out common R-Car Gen3 bias handling

All pin control drivers for R-Car Gen3 SoCs contain identical bias
handling.  Reduce code duplication by moving it to the common pinctrl.c
code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201028151637.1734130-6-geert+renesas@glider.be