dt-bindings: nvmem: Add STM32 factory-programmed romem

Add documentation for STMicroelectronics STM32 Factory-programmed
read only memory area.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-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/st,stm32-romem.txt

+STMicroelectronics STM32 Factory-programmed data device tree bindings
+
+This represents STM32 Factory-programmed read only non-volatile area: locked
+flash, OTP, read-only HW regs... This contains various information such as:
+analog calibration data for temperature sensor (e.g. TS_CAL1, TS_CAL2),
+internal vref (VREFIN_CAL), unique device ID...
+
+Required properties:
+- compatible:		Should be one of:
+			"st,stm32f4-otp"
+			"st,stm32mp15-bsec"
+- reg:			Offset and length of factory-programmed area.
+- #address-cells:	Should be '<1>'.
+- #size-cells:		Should be '<1>'.
+
+Optional Data cells:
+- Must be child nodes as described in nvmem.txt.
+
+Example on stm32f4:
+	romem: nvmem@1fff7800 {
+		compatible = "st,stm32f4-otp";
+		reg = <0x1fff7800 0x400>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		/* Data cells: ts_cal1 at 0x1fff7a2c */
+		ts_cal1: calib@22c {
+			reg = <0x22c 0x2>;
+		};
+		...
+	};