spi: stm32_qspi: use QSPI bus as 8 lines communication channel

Merge series from patrice.chotard@foss.st.com <patrice.chotard@foss.st.com>:

The goal of this series is to allow to use QSPI bus as a 8 lines communication
channel for specific purpose.

The QSPI block offers the possibility to communicate with 2 flashes in
parrallel using the dual flash mode, 8 data lines are then used.
Usage of cs-gpios populated and spi-tx-bus-width / spi-rx-bus-width both set to 8,
is needed to enable dual flash mode.

The addition of the legacy transfer_one_message() spi callback is also needed
as currently the stm32-qspi driver only supports spi_controller_mem_ops API.