dt-bindings: mailbox: add STMicroelectronics STM32 IPCC binding

Add a binding for the STMicroelectronics STM32 IPCC block exposing a
mailbox mechanism between two processors.
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/stm32-ipcc.txt

+* STMicroelectronics STM32 IPCC (Inter-Processor Communication Controller)
+
+The IPCC block provides a non blocking signaling mechanism to post and
+retrieve messages in an atomic way between two processors.
+It provides the signaling for N bidirectionnal channels. The number of channels
+(N) can be read from a dedicated register.
+
+Required properties:
+- compatible:   Must be "st,stm32mp1-ipcc"
+- reg:          Register address range (base address and length)
+- st,proc-id:   Processor id using the mailbox (0 or 1)
+- clocks:       Input clock
+- interrupt-names: List of names for the interrupts described by the interrupt
+                   property. Must contain the following entries:
+                   - "rx"
+                   - "tx"
+                   - "wakeup"
+- interrupts:   Interrupt specifiers for "rx channel occupied", "tx channel
+                free" and "system wakeup".
+- #mbox-cells:  Number of cells required for the mailbox specifier. Must be 1.
+                The data contained in the mbox specifier of the "mboxes"
+                property in the client node is the mailbox channel index.
+
+Optional properties:
+- wakeup-source: Flag to indicate whether this device can wake up the system
+
+
+
+Example:
+	ipcc: mailbox@4c001000 {
+		compatible = "st,stm32mp1-ipcc";
+		#mbox-cells = <1>;
+		reg = <0x4c001000 0x400>;
+		st,proc-id = <0>;
+		interrupts-extended = <&intc GIC_SPI 100 IRQ_TYPE_NONE>,
+				      <&intc GIC_SPI 101 IRQ_TYPE_NONE>,
+				      <&aiec 62 1>;
+		interrupt-names = "rx", "tx", "wakeup";
+		clocks = <&rcc_clk IPCC>;
+		wakeup-source;
+	}
+
+Client:
+	mbox_test {
+		...
+		mboxes = <&ipcc 0>, <&ipcc 1>;
+	};