spi: mediatek: add bindings for Mediatek MT2712 soc platform

This patch adds a DT binding documentation for the MT2712 soc.
Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-slave-mt27xx.txt

+Binding for MTK SPI Slave controller
+
+Required properties:
+- compatible: should be one of the following.
+    - mediatek,mt2712-spi-slave: for mt2712 platforms
+- reg: Address and length of the register set for the device.
+- interrupts: Should contain spi interrupt.
+- clocks: phandles to input clocks.
+  It's clock gate, and should be <&infracfg CLK_INFRA_AO_SPI1>.
+- clock-names: should be "spi" for the clock gate.
+
+Optional properties:
+- assigned-clocks: it's mux clock, should be <&topckgen CLK_TOP_SPISLV_SEL>.
+- assigned-clock-parents: parent of mux clock.
+  It's PLL, and should be one of the following.
+   -  <&topckgen CLK_TOP_UNIVPLL1_D2>: specify parent clock 312MHZ.
+				       It's the default one.
+   -  <&topckgen CLK_TOP_UNIVPLL1_D4>: specify parent clock 156MHZ.
+   -  <&topckgen CLK_TOP_UNIVPLL2_D4>: specify parent clock 104MHZ.
+   -  <&topckgen CLK_TOP_UNIVPLL1_D8>: specify parent clock 78MHZ.
+
+Example:
+- SoC Specific Portion:
+spis1: spi@10013000 {
+	compatible = "mediatek,mt2712-spi-slave";
+	reg = <0 0x10013000 0 0x100>;
+	interrupts = <GIC_SPI 283 IRQ_TYPE_LEVEL_LOW>;
+	clocks = <&infracfg CLK_INFRA_AO_SPI1>;
+	clock-names = "spi";
+	assigned-clocks = <&topckgen CLK_TOP_SPISLV_SEL>;
+	assigned-clock-parents = <&topckgen CLK_TOP_UNIVPLL1_D2>;
+};