nvmem: dt: document SNVS LPGPR binding

Documentation bindings for the Low Power General Purpose Register
available on i.MX6 SoCs in the Secure Non-Volatile Storage.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/nvmem/snvs-lpgpr.txt

+Device tree bindings for Low Power General Purpose Register found in i.MX6Q/D
+Secure Non-Volatile Storage.
+
+This DT node should be represented as a sub-node of a "syscon",
+"simple-mfd" node.
+
+Required properties:
+- compatible: should be one of the fallowing variants:
+	"fsl,imx6q-snvs-lpgpr" for Freescale i.MX6Q/D/DL/S
+	"fsl,imx6ul-snvs-lpgpr" for Freescale i.MX6UL
+
+Example:
+snvs: snvs@020cc000 {
+	compatible = "fsl,sec-v4.0-mon", "syscon", "simple-mfd";
+	reg = <0x020cc000 0x4000>;
+
+	snvs_lpgpr: snvs-lpgpr {
+		compatible = "fsl,imx6q-snvs-lpgpr";
+	};
+};