ARM: dts: rza2mevb: Add input switch

Add support for input switch SW3 on the Renesas RZ/A2M EVB
development board.

Note that this uses the IRQ interrupt, as the RZ/A2 GPIO controller
does not include interrupt support

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>