dt-bindings: mailbox: add support for mailbox client shared memory

Many users of the mailbox controllers depend on the shared memory
between the two end points to exchange the main data while using simple
doorbell mechanism to alert the end points of the presence of a message.

This patch defines device tree bindings to represent such shared memory
in a generic way.

Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

Documentation/devicetree/bindings/mailbox/mailbox.txt

@@ -23,6 +23,11 @@ Required property:
 
 Optional property:
 - mbox-names: List of identifier strings for each mailbox channel.
+- shmem : List of phandle pointing to the shared memory(SHM) area between the
+	  users of these mailboxes for IPC, one for each mailbox. This shared
+	  memory can be part of any memory reserved for the purpose of this
+	  communication between the mailbox client and the remote.
+
 
 Example:
 	pwr_cntrl: power {
@@ -30,3 +35,26 @@ Example:
 		mbox-names = "pwr-ctrl", "rpc";
 		mboxes = <&mailbox 0 &mailbox 1>;
 	};
+
+Example with shared memory(shmem):
+
+	sram: sram@50000000 {
+		compatible = "mmio-sram";
+		reg = <0x50000000 0x10000>;
+
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges = <0 0x50000000 0x10000>;
+
+		cl_shmem: shmem@0 {
+			compatible = "client-shmem";
+			reg = <0x0 0x200>;
+		};
+	};
+
+	client@2e000000 {
+		...
+		mboxes = <&mailbox 0>;
+		shmem = <&cl_shmem>;
+		..
+	};