arm64: dts: renesas: gray-hawk-single: Enable nfsroot

Extend the default kernel command line for Gray Hawk Single for mounting
the root filesystem via NFS, like is done for all other Renesas
development boards.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/0c291919071350674648317f8cf3efbd1ca4475f.1712068442.git.geert+renesas@glider.be

arch/arm64/boot/dts/renesas/r8a779h0-gray-hawk-single.dts

@@ -23,7 +23,7 @@ aliases {
 	};
 
 	chosen {
-		bootargs = "ignore_loglevel";
+		bootargs = "ignore_loglevel rw root=/dev/nfs ip=on";
 		stdout-path = "serial0:921600n8";
 	};