spi: update spi slave bindings for MT8195 SoC

Add a DT binding documentation for the MT8195 soc.
Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Link: https://lore.kernel.org/r/20210322055244.30179-3-leilk.liu@mediatek.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -3,6 +3,7 @@ Binding for MTK SPI Slave controller
 Required properties:
 - compatible: should be one of the following.
     - mediatek,mt2712-spi-slave: for mt2712 platforms
+    - mediatek,mt8195-spi-slave: for mt8195 platforms
 - reg: Address and length of the register set for the device.
 - interrupts: Should contain spi interrupt.
 - clocks: phandles to input clocks.