ARM: dts: r8a779x: Configure PMIC IRQ pinmux

The PMIC IRQ line pin multiplexing configuration is missing from the DTs.
Since the line is configured correctly by default, the system works fine.
However, add the IRQ line pin multiplexing configuration for completeness.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>