arm64: dts: renesas: Factor out Ebisu board support

Move the common parts for the Renesas Ebisu board to ebisu.dtsi, to
avoid future duplication of board descriptions.

Change a reference in a comment from "r8a77990" to "R-Car E3(e)", to
prepare for the advent of "R-Car E3e".
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/df275772dea0a4fa88d1bffa96ce048eaa7d5308.1628766192.git.geert+renesas@glider.be