MLK-14498-5 tty: serial: imx: fix the DMA issue

MLK-14498-5 tty: serial: imx: fix the DMA issue

The commmunity driver uart DMA don't work, it better to use
4.1.y DMA process mechanism, so there have many conflict during
code merging. Decisively, to use 4.1.y commit f00cf8855eaa in the
merge point for DMA implemention.

In DMA mode, don't involve CPU interrupt, remove .imx_dma_rxint()
function.

After the patch, DMA and CPU mode both work fine with the current
SDMA driver.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>