spi: stm32_qspi: move to exec_op

spi: stm32_qspi: move to exec_op

We are facing issues in the driver since SPI NOR framework has moved
on SPI MEM framework, and SPI NAND framework is not running properly
with the current driver.

To be able to solve issues met on SPI NOR Flashes and to be able to
support SPI NAND Flashes, the driver has been reworked. We are now using
exec_op ops instead of using xfer ops.

Thanks to this rework, the driver has been successfully tested with:
 - mx66l51235l SPI NOR Flash on stm32f746 SOC
 - n25q128a SPI NOR Flash on stm32f769 SOC
 - mx66l51235l SPI NOR Flash on stm32mp1 SOC
 - mt29f2g01abagd SPI NAND Flash on stm32mp1 SOC

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Tested-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>