drivers: spi: smartbond: remove default enabling of dma acceleration

The Smartbond SPI driver enables DMA acceleration by default. However
no DMA configuration is provided in the SPI node, resulting in run
time errors when the SPI interface is used. Instead, the application
should explicitly enable DMA acceleration and select the DMA channels
and priorities.

Signed-off-by: Ian Morris <ian.d.morris@outlook.com>