sh-pfc: sh73a0: Add missing IRQ15

The external IRQ15 input multiplexed on GPIO 0 is missing. Add it.

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