ARM: dts: stm32: Configure qspi's mdma transfer to block for stm32mp151

Configure qspi's mdma from buffer transfer (max 128 bytes) to
block transfer (max 64K bytes).

mtd_speedtest shows that write throughtput increases :
  - from 734 to 782 KiB/s (~6.5%) with s25fl512s SPI-NOR.
  - from 4848 to 5319 KiB/s (~9.72%) with Micron SPI-NAND.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>