pinctrl: sh-pfc: r8a77965: Add support for INTC-EX IRQ pins

Most pins on the R8A77965 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 R8A77965 the INTC-EX hardware handles pins IRQ0 -> IRQ5 and
this patch adds support for them to the PFC driver as "intc_ex_irqN".

Based on a similar patch for the R8A7795 PFC driver by Magnus Damm
<damm+renesas@opensource.se>.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>