arm64: dts: renesas: Factor out White Hawk Single board support

Move the common parts for the Renesas White Hawk Single board to
white-hawk-single.dtsi, to enable future reuse.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/1661743b18a9ff9fac716f98a663b39fc8488d7e.1733156661.git.geert+renesas@glider.be