sh-pfc: Look up IRQ table entries by GPIO number

Instead of converting the GPIO number to an enum_id and looking up IRQ
table entries by enum_id, replace the pinmux_irq enum_ids field with a
gpios field and lookup entries using the GPIO number.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>