drivers: pinctrl: Add R-Car Gen4 support

Renesas R-Car Gen4 is different from Gen3 regarding pinmux.

While Gen3 had only one base address to manage all pins,
Gen4 has one set of pinmux registers per GPIO banks.

We could expose one pinmux register per GPIO controllers,
but that would break potential compatibility with Linux
Device tree.

Instead create a reg_base array to parse all reg base from
device tree and identify proper base address based on the pin
definition.

This imply to add a pfc_base parameter to most of the pfc_rcar
function.

Signed-off-by: Julien Massot <julien.massot@iot.bzh>
Signed-off-by: Pierre Marzin <pierre.marzin@iot.bzh>
Signed-off-by: Aymeric Aillet <aymeric.aillet@iot.bzh>