mach-stm32: Add MPU region for spi-nor memory mapped region

The Quad-SPI interface is able to manage up to 256Mbytes Flash
memory starting from 0x90000000 to 0x9FFFFFFF in the memory
mapped mode.
Add a dedicated MPU region into stm32_region_config.

See application note AN4760 available at www.st.com

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>