dt-bindings: spi: add binding file for NXP FlexSPI controller

Add binding file for NXP FlexSPI controller
Signed-off-by: Yogesh Narayan Gaur <yogeshnarayan.gaur@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-nxp-fspi.txt

+* NXP Flex Serial Peripheral Interface (FSPI)
+
+Required properties:
+  - compatible : Should be "nxp,lx2160a-fspi"
+  - reg :        First contains the register location and length,
+                 Second contains the memory mapping address and length
+  - reg-names :  Should contain the resource reg names:
+	         - fspi_base: configuration register address space
+                 - fspi_mmap: memory mapped address space
+  - interrupts : Should contain the interrupt for the device
+
+Required SPI slave node properties:
+  - reg :        There are two buses (A and B) with two chip selects each.
+                 This encodes to which bus and CS the flash is connected:
+                 - <0>: Bus A, CS 0
+                 - <1>: Bus A, CS 1
+                 - <2>: Bus B, CS 0
+                 - <3>: Bus B, CS 1
+
+Example showing the usage of two SPI NOR slave devices on bus A:
+
+fspi0: spi@20c0000 {
+	compatible = "nxp,lx2160a-fspi";
+	reg = <0x0 0x20c0000 0x0 0x10000>, <0x0 0x20000000 0x0 0x10000000>;
+	reg-names = "fspi_base", "fspi_mmap";
+	interrupts = <0 25 0x4>; /* Level high type */
+	clocks = <&clockgen 4 3>, <&clockgen 4 3>;
+	clock-names = "fspi_en", "fspi";
+
+	mt35xu512aba0: flash@0 {
+		reg = <0>;
+		....
+	};
+
+	mt35xu512aba1: flash@1 {
+		reg = <1>;
+		....
+	};
+};