pinctrl: renesas: Make sh_pfc_pin_to_bias_reg() static

Now all R-Car pin control drivers have been converted to the common
R-Car bias handling, sh_pfc_pin_to_bias_reg() is only called from a
single place.  Move it from core.c to pinctrl.c, make it static, and
rename it to rcar_pin_to_bias_reg(), as it is specific to R-Car SoCs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20210303132619.3938128-2-geert+renesas@glider.be