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

spi: stm32-ospi: 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.

Running mtd_speedtest on a 4MB sNOR partition using a
stm32mp257f-ev1 board gives the following results:

           before        after   gain
Read :  5693 KiB/s  21139 KiB/s   371%
Write:   765 KiB/s    910 KiB/s    19%

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://patch.msgid.link/20251208-upstream_qspi_ospi_updates-v2-2-62526c9467dc@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>