nvmem: Add driver for STM32 factory-programmed read only mem

Add a read only nvmem driver for STM32 factory-programmed memory area
(on-chip non-volatile storage).

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>