arm64: dts: renesas: white-hawk-cpu: Add push switches

Describe the three Push Switches on the White Hawk CPU board, so
they can be used for user input.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/e18d0d5087a514db611295f5d1e13c950cf7dae7.1662715538.git.geert+renesas@glider.be