spi: stm32_qspi: Add chip select management

Quad-SPI interface is able to manage 2 spi nor devices.
FSEL bit selects the flash memory to be addressed in single flash mode.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>