drivers: flash: octo spi for stm32 with DMA

Introducing the dma transfer (also through dmamux)
to transfer data to/from the NOR octo-flash
With a DMAMUX, the DMA channel is given by the DTS.
Note that STM32U5X does not support DMA here.

Signed-off-by: Francois Ramu <francois.ramu@st.com>