pinctrl: sh-pfc: r8a7791: Add I2C pins

This adds I2C[0-4] pinmux support to R8A7791 SoC.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>