dt-bindings: stm32: add bindings for ML-AHB interconnect

Document the ML-AHB interconnect for stm32 SoCs.
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/arm/stm32/mlahb.txt

+ML-AHB interconnect bindings
+
+These bindings describe the STM32 SoCs ML-AHB interconnect bus which connects
+a Cortex-M subsystem with dedicated memories.
+The MCU SRAM and RETRAM memory parts can be accessed through different addresses
+(see "RAM aliases" in [1]) using different buses (see [2]) : balancing the
+Cortex-M firmware accesses among those ports allows to tune the system
+performance.
+
+[1]: https://www.st.com/resource/en/reference_manual/dm00327659.pdf
+[2]: https://wiki.st.com/stm32mpu/wiki/STM32MP15_RAM_mapping
+
+Required properties:
+- compatible: should be "simple-bus"
+- dma-ranges: describes memory addresses translation between the local CPU and
+	   the remote Cortex-M processor. Each memory region, is declared with
+	   3 parameters:
+		 - param 1: device base address (Cortex-M processor address)
+		 - param 2: physical base address (local CPU address)
+		 - param 3: size of the memory region.
+
+The Cortex-M remote processor accessed via the mlahb interconnect is described
+by a child node.
+
+Example:
+mlahb {
+	compatible = "simple-bus";
+	#address-cells = <1>;
+	#size-cells = <1>;
+	dma-ranges = <0x00000000 0x38000000 0x10000>,
+		     <0x10000000 0x10000000 0x60000>,
+		     <0x30000000 0x30000000 0x60000>;
+
+	m4_rproc: m4@10000000 {
+		...
+	};
+};