ARM: shmobile: lager: Specify external clock frequency in DT

The external crystal frequency is 20MHz on the Lager board. Specify it
in the device tree.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/r8a7790-lager.dts

@@ -58,6 +58,10 @@ fixedregulator3v3: fixedregulator@0 {
 	};
 };
 
+&extal_clk {
+	clock-frequency = <20000000>;
+};
+
 &pfc {
 	pinctrl-0 = <&scif0_pins &scif1_pins>;
 	pinctrl-names = "default";