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>