ARM: scu: document Snoop Control Unit DT bindings

This adds device tree bindings for the ARM Cortex-A5 and
Cortex-A9 Snoop Control Units.
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/arm/scu.txt

+* ARM Snoop Control Unit (SCU)
+
+As part of the MPCore complex, Cortex-A5 and Cortex-A9 are provided
+with a Snoop Control Unit. The register range is usually 256 (0x100)
+bytes.
+
+References:
+
+- Cortex-A9: see DDI0407E Cortex-A9 MPCore Technical Reference Manual
+  Revision r2p0
+- Cortex-A5: see DDI0434B Cortex-A5 MPCore Technical Reference Manual
+  Revision r0p1
+
+- compatible : Should be:
+	"arm,cortex-a9-scu"
+	"arm,cortex-a5-scu"
+
+- reg : Specify the base address and the size of the SCU register window.
+
+Example:
+
+scu@a04100000 {
+	compatible = "arm,cortex-a9-scu";
+	reg = <0xa0410000 0x100>;
+};