pinctrl: sh-pfc: r8a7795: Add support for INTC-EX IRQ pins

Most pins on the r8a7795 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".

Tested on r8a7795 Salvator-X with an external loop back adapter on
EXIO_D that connects pin 9 (IRQ2/GP2_02) and pin 26 (ExA22/GP2_06).

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>