mxs: spi: Remove CONFIG_MXS_SPI_DMA_ENABLE

The CONFIG_MXS_SPI_DMA_ENABLE is no longer relevant as the SPI DMA
has proven to work correctly. Remove this configuration option.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>