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>;
};