arm64: dts: renesas: r8a77990: ebisu: Add SW4 support

Add support for SW4 via "gpio-keys" on the R-Car E3 Ebisu board.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Link: https://lore.kernel.org/r/162416649714.11444.12659951671817146236.sendpatchset@octo


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>