dt-bindings: Add a binding for Mediatek SCP

Add a DT binding documentation of SCP for the
MT8183 SoC from Mediatek.
Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191112110330.179649-2-pihsun@chromium.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Documentation/devicetree/bindings/remoteproc/mtk,scp.txt

+Mediatek SCP Bindings
+----------------------------------------
+
+This binding provides support for ARM Cortex M4 Co-processor found on some
+Mediatek SoCs.
+
+Required properties:
+- compatible		Should be "mediatek,mt8183-scp"
+- reg			Should contain the address ranges for the two memory
+			regions, SRAM and CFG.
+- reg-names		Contains the corresponding names for the two memory
+			regions. These should be named "sram" & "cfg".
+- clocks		Clock for co-processor (See: ../clock/clock-bindings.txt)
+- clock-names		Contains the corresponding name for the clock. This
+			should be named "main".
+
+Subnodes
+--------
+
+Subnodes of the SCP represent rpmsg devices. The names of the devices are not
+important. The properties of these nodes are defined by the individual bindings
+for the rpmsg devices - but must contain the following property:
+
+- mtk,rpmsg-name	Contains the name for the rpmsg device. Used to match
+			the subnode to rpmsg device announced by SCP.
+
+Example:
+
+	scp: scp@10500000 {
+		compatible = "mediatek,mt8183-scp";
+		reg = <0 0x10500000 0 0x80000>,
+		      <0 0x105c0000 0 0x5000>;
+		reg-names = "sram", "cfg";
+		clocks = <&infracfg CLK_INFRA_SCPSYS>;
+		clock-names = "main";
+	};