dmaengine: Kconfig: Simplify the help text for MXS_DMA

Currently the help text for the MXS_DMA option is incomplete as it does
not mention MX6SX, MX6ULL and MX7D, for example.

Instead of extending this list everytime a new SoC comes out, let's
keep the text more generic.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>