ARM: dts: stm32: Update part number NVMEM description on stm32mp131

The STM32MP13x Device Part Number (also named RPN in reference manual)
only uses the first 12 bits in OTP4, all the other bit are reserved and
they can be different of zero; they must be masked in NVMEM result, so
the number of bits must be defined in the nvmem cell description.

Fixes: 1da8779c ("ARM: dts: stm32: add STM32MP13 SoCs support")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>