pinctrl: renesas: r8a77970: Add support for AVB power-source

Add support for configuring the I/O voltage level of the Ethernet AVB
pins on the R-Car V3M SoC.  "PIN_VDDQ_AVB0" can be configured for 2.5V
or 3.3V operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/eb4db82bdeb67507a1a77f63b9d90280d6f38ba4.1678271030.git.geert+renesas@glider.be