arm64: dts: renesas: Add common Salvator-XS board support

The Renesas Salvator-XS (Salvator-X 2nd version) development board can
be equipped with either an R-Car H3 ES2.0 or M3-W ES1.x SiP, which are
pin-compatible.

Add initial support for the common parts of the Salvator-XS board into
its own .dtsi file, to be included by the DTSes for the H3/M3-W
versions.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/salvator-xs.dtsi

+/*
+ * Device Tree Source for the Salvator-X 2nd version board
+ *
+ * Copyright (C) 2015-2017 Renesas Electronics Corp.
+ *
+ * This file is licensed under the terms of the GNU General Public License
+ * version 2.  This program is licensed "as is" without any warranty of any
+ * kind, whether express or implied.
+ */
+
+#include "salvator-common.dtsi"
+
+/ {
+	model = "Renesas Salvator-X 2nd version board";
+	compatible = "renesas,salvator-xs";
+};
+
+&extal_clk {
+	clock-frequency = <16640000>;
+};