ARM: dts: stm32: add part number for STM32MP15x

The STM32MP15x Device Part Number is located in the first 8 bits of OTP4,
this patch add its description as the NVMEM cell.
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

arch/arm/boot/dts/stm32mp151.dtsi

@@ -1620,6 +1620,9 @@ bsec: efuse@5c005000 {
 			reg = <0x5c005000 0x400>;
 			#address-cells = <1>;
 			#size-cells = <1>;
+			part_number_otp: part-number-otp@4 {
+				reg = <0x4 0x1>;
+			};
 			ts_cal1: calib@5c {
 				reg = <0x5c 0x2>;
 			};