Documentation: devicetree: add Broadcom GISB arbiter bindings

This patch adds the Broadcom GISB arbiter Device Tree binding that is
used on all Broadcom Set-top-box System-on-a-chip.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/bus/brcm,gisb-arb.txt

+Broadcom GISB bus Arbiter controller
+
+Required properties:
+
+- compatible: should be "brcm,gisb-arb"
+- reg: specifies the base physical address and size of the registers
+- interrupt-parent: specifies the phandle to the parent interrupt controller
+  this arbiter gets interrupt line from
+- interrupts: specifies the two interrupts (timeout and TEA) to be used from
+  the parent interrupt controller
+
+Optional properties:
+
+- brcm,gisb-arb-master-mask: 32-bits wide bitmask used to specify which GISB
+  masters are valid at the system level
+- brcm,gisb-arb-master-names: string list of the litteral name of the GISB
+  masters. Should match the number of bits set in brcm,gisb-master-mask and
+  the order in which they appear
+
+Example:
+
+gisb-arb@f0400000 {
+	compatible = "brcm,gisb-arb";
+	reg = <0xf0400000 0x800>;
+	interrupts = <0>, <2>;
+	interrupt-parent = <&sun_l2_intc>;
+
+	brcm,gisb-arb-master-mask = <0x7>;
+	brcm,gisb-arb-master-names = "bsp_0", "scpu_0", "cpu_0";
+};