dt-bindings: pwm: Add i.MX TPM PWM binding

Add i.MX TPM(Low Power Timer/Pulse Width Modulation Module) PWM binding.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/imx-tpm-pwm.txt

+Freescale i.MX TPM PWM controller
+
+Required properties:
+- compatible : Should be "fsl,imx7ulp-pwm".
+- reg: Physical base address and length of the controller's registers.
+- #pwm-cells: Should be 3. See pwm.txt in this directory for a description of the cells format.
+- clocks : The clock provided by the SoC to drive the PWM.
+- interrupts: The interrupt for the PWM controller.
+
+Note: The TPM counter and period counter are shared between multiple channels, so all channels
+should use same period setting.
+
+Example:
+
+tpm4: pwm@40250000 {
+	compatible = "fsl,imx7ulp-pwm";
+	reg = <0x40250000 0x1000>;
+	assigned-clocks = <&pcc2 IMX7ULP_CLK_LPTPM4>;
+	assigned-clock-parents = <&scg1 IMX7ULP_CLK_SOSC_BUS_CLK>;
+	clocks = <&pcc2 IMX7ULP_CLK_LPTPM4>;
+	#pwm-cells = <3>;
+};