dt-bindings: pinctrl: renesas,pfc: Document R-Car V4M support

Document support for the Pin Function Controller (PFC) in the Renesas
R-Car V4M (R8A779H0) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/56685dc04af3cee7cb3751e855ed5b3679b14122.1706264667.git.geert+renesas@glider.be