pinctrl: sh-pfc: Don't set the pinmux_irq irq field for multiplatform

In the multiplatform kernel case the IRQs associated with the PFC GPIOs
are specified through DT. The pinmux_irq irq field is thus ignored by
the code, and doesn't need to be set.

This will allow removing the mach/irq.h include from pfc-*.c files that
was required for the irq_pin() macro used to initialize the irq field.

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