DEVICETREE: Add bindings for the SoC of the ATH79 family

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/mips/ath79-soc.txt

+Binding for Qualcomm Atheros AR7xxx/AR9XXX SoC
+
+Each device tree must specify a compatible value for the AR SoC
+it uses in the compatible property of the root node. The compatible
+value must be one of the following values:
+
+- qca,ar7130
+- qca,ar7141
+- qca,ar7161
+- qca,ar7240
+- qca,ar7241
+- qca,ar7242
+- qca,ar9130
+- qca,ar9132
+- qca,ar9330
+- qca,ar9331
+- qca,ar9341
+- qca,ar9342
+- qca,ar9344
+- qca,qca9556
+- qca,qca9558