spi: imx: add devicetree binding for lpspi

Add a binding document for lpspi driver
Signed-off-by: Gao Pan <pandy.gao@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-fsl-lpspi.txt

+* Freescale Low Power SPI (LPSPI) for i.MX
+
+Required properties:
+- compatible :
+  - "fsl,imx7ulp-spi" for LPSPI compatible with the one integrated on i.MX7ULP soc
+- reg : address and length of the lpspi master registers
+- interrupt-parent : core interrupt controller
+- interrupts : lpspi interrupt
+- clocks : lpspi clock specifier
+
+Examples:
+
+lpspi2: lpspi@40290000 {
+	compatible = "fsl,imx7ulp-spi";
+	reg = <0x40290000 0x10000>;
+	interrupt-parent = <&intc>;
+	interrupts = <GIC_SPI 28 IRQ_TYPE_LEVEL_HIGH>;
+	clocks = <&clks IMX7ULP_CLK_LPSPI2>;
+};