spi: meson: Add device tree bindings documentation for SPIFC

This adds documentation of device tree bindings for the Amlogic Meson
SPIFC (SPI Flash Controller).
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-meson.txt

+Amlogic Meson SPI controllers
+
+* SPIFC (SPI Flash Controller)
+
+The Meson SPIFC is a controller optimized for communication with SPI
+NOR memories, without DMA support and a 64-byte unified transmit /
+receive buffer.
+
+Required properties:
+ - compatible: should be "amlogic,meson6-spifc"
+ - reg: physical base address and length of the controller registers
+ - clocks: phandle of the input clock for the baud rate generator
+ - #address-cells: should be 1
+ - #size-cells: should be 0
+
+	spi@c1108c80 {
+		compatible = "amlogic,meson6-spifc";
+		reg = <0xc1108c80 0x80>;
+		clocks = <&clk81>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+	};