dt-bindings: remoteproc: add bindings for stm32 remote processor driver

Add the device tree bindings document for the stm32 remoteproc devices.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Documentation/devicetree/bindings/remoteproc/stm32-rproc.txt

+STMicroelectronics STM32 Remoteproc
+-----------------------------------
+This document defines the binding for the remoteproc component that loads and
+boots firmwares on the ST32MP family chipset.
+
+Required properties:
+- compatible:	Must be "st,stm32mp1-m4"
+- reg:		Address ranges of the RETRAM and MCU SRAM memories used by the
+		remote processor.
+- resets:	Reference to a reset controller asserting the remote processor.
+- st,syscfg-holdboot: Reference to the system configuration which holds the
+		remote processor reset hold boot
+	1st cell: phandle of syscon block
+	2nd cell: register offset containing the hold boot setting
+	3rd cell: register bitmask for the hold boot field
+- st,syscfg-tz: Reference to the system configuration which holds the RCC trust
+		zone mode
+	1st cell: phandle to syscon block
+	2nd cell: register offset containing the RCC trust zone mode setting
+	3rd cell: register bitmask for the RCC trust zone mode bit
+
+Optional properties:
+- interrupts:	Should contain the watchdog interrupt
+- mboxes:	This property is required only if the rpmsg/virtio functionality
+		is used. List of phandle and mailbox channel specifiers:
+		- a channel (a) used to communicate through virtqueues with the
+		  remote proc.
+		  Bi-directional channel:
+		      - from local to remote = send message
+		      - from remote to local = send message ack
+		- a channel (b) working the opposite direction of channel (a)
+		- a channel (c) used by the local proc to notify the remote proc
+		  that it is about to be shut down.
+		  Unidirectional channel:
+		      - from local to remote, where ACK from the remote means
+		        that it is ready for shutdown
+- mbox-names:	This property is required if the mboxes property is used.
+		- must be "vq0" for channel (a)
+		- must be "vq1" for channel (b)
+		- must be "shutdown" for channel (c)
+- memory-region: List of phandles to the reserved memory regions associated with
+		the remoteproc device. This is variable and describes the
+		memories shared with the remote processor (eg: remoteproc
+		firmware and carveouts, rpmsg vrings, ...).
+		(see ../reserved-memory/reserved-memory.txt)
+- st,syscfg-pdds: Reference to the system configuration which holds the remote
+		processor deep sleep setting
+	1st cell: phandle to syscon block
+	2nd cell: register offset containing the deep sleep setting
+	3rd cell: register bitmask for the deep sleep bit
+- st,auto-boot:	If defined, when remoteproc is probed, it loads the default
+		firmware and starts the remote processor.
+
+Example:
+	m4_rproc: m4@10000000 {
+		compatible = "st,stm32mp1-m4";
+		reg = <0x10000000 0x40000>,
+		      <0x30000000 0x40000>,
+		      <0x38000000 0x10000>;
+		resets = <&rcc MCU_R>;
+		st,syscfg-holdboot = <&rcc 0x10C 0x1>;
+		st,syscfg-tz = <&rcc 0x000 0x1>;
+	};