arm: rmobile: Restrict PINCTRL_PFC selection to R-Car gen3/gen4

The RZ/G2L family uses CONFIG_RCAR_64 but does not share a common PFC
driver with the R-Car gen3 & gen4 boards.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>