gpio: rcar: Remove redundant compatible values

The mandatory compatible values 'renesas,rcar-gen{1,2,3}-gpio' have been
already added to all the respective R-Car Gen{1,2,3} SoC DTSI files,
remove the redundant device specific values from the driver.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>