pinctrl: sh-pfc: r8a7796: Add support for INTC-EX IRQ pins

Most pins on the r8a7796 SoC can be configured in GPIO mode for
interrupt and GPIO functionality, while a couple of them can also
be routed to the INTC-EX hardware block (formerly known as IRQC).

On r8a7795 the INTC-EX hardware handles pins IRQ0 -> IRQ5 and
this patch adds support for them to the PFC driver as "intc_ex_irqN".

[takeshi.kihara.df: Ported from commit bb46f6f3 ("pinctrl: sh-pfc:
 r8a7795: Add support for INTC-EX IRQ pins")
 to drivers/pinctrl/sh-pfc/pfc-r8a7796.c]
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>