ARM: shmobile: henninger: specify EXTAL frequency

When creating the initial device tree for the Henninger board,  I've overlooked
that EXTAL frequency needs to be overridden there. The 'sh-sci' driver  managed
to work somehow but the SDHI driver that I've tried to enable just hanged with
the default EXTAL frequency of 0...
Signed-off-by: NSergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/r8a7791-henninger.dts

@@ -35,6 +35,10 @@
 	};
 };
 
+&extal_clk {
+	clock-frequency = <20000000>;
+};
+
 &pfc {
 	scif0_pins: serial0 {
 		renesas,groups = "scif0_data_d";