mailbox: dt: Supply bindings for ST's Mailbox IP

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/sti-mailbox.txt

+ST Microelectronics Mailbox Driver
+
+Each ST Mailbox IP currently consists of 4 instances of 32 channels.  Messages
+are passed between Application and Remote processors using shared memory.
+
+Controller
+----------
+
+Required properties:
+- compatible		: Should be "st,stih407-mailbox"
+- reg			: Offset and length of the device's register set
+- mbox-name		: Name of the mailbox
+- #mbox-cells:		: Must be 2
+			  <&phandle instance channel direction>
+			    phandle   : Label name of controller
+			    instance  : Instance number
+			    channel   : Channel number
+
+Optional properties
+- interrupts		: Contains the IRQ line for a Rx mailbox
+
+Example:
+
+mailbox0: mailbox@0  {
+	compatible	= "st,stih407-mailbox";
+	reg		= <0x08f00000 0x1000>;
+	interrupts	= <GIC_SPI 1 IRQ_TYPE_NONE>;
+	#mbox-cells	= <2>;
+	mbox-name	= "a9";
+};
+
+Client
+------
+
+Required properties:
+- compatible		: Many (See the client docs)
+- reg			: Shared (between Application and Remote) memory address
+- mboxes		: Standard property to specify a Mailbox (See ./mailbox.txt)
+			  Cells must match 'mbox-cells' (See Controller docs above)
+
+Optional properties
+- mbox-names		: Name given to channels seen in the 'mboxes' property.
+
+Example:
+
+mailbox_test {
+	compatible	= "mailbox_test";
+	reg		= <0x[shared_memory_address], [shared_memory_size]>;
+	mboxes		= <&mailbox2 0 1>, <&mailbox0 2 1>;
+	mbox-names	= "tx",	"rx";
+};