drivers: spi: stm32h7 Fix SPI DMA 16 bits frames

Modify STM32H7 SPI driver so that it updates the rx/tx
pointers correctly (depending on the frame size) when DMA
is enabled. Also, make the dummy rx/tx buffer cache-coherent.

Signed-off-by: Daniel Gaston Ochoa <dgastonochoa@gmail.com>