spi: stm32-qspi: Optimize FIFO accesses using u16 or u32

spi: stm32-qspi: Optimize FIFO accesses using u16 or u32

FIFO accesses uses u8 only for read/write.
In order to optimize throughput, add u16 or u32 read/write
accesses when possible.
Set FIFO threshold level value accordingly.

Test performed by writing and reading 64MB on sNOR on
stm32mp157c-ev1 board:

          before      after    ratio
Write :  428 KB/s   719 KB/s    +68%
Read  :  520 KB/s  3200 KB/s   +615%

Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>