arm64: dts: renesas: white-hawk-cpu: Add GP LEDs

Describe the three General Purpose LEDs on the White Hawk CPU board, so
they can be used as indicator LEDs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/007acd941ef925057f1f9b925ed4e339dbd29a74.1662715538.git.geert+renesas@glider.be