dt-bindings: mailbox: add STMicroelectronics STM32 IPCC binding

* 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>;

	};