MFC r358450, r358635

r358450:
mmc: dwmmc: Fix off by one error

The IVAR_MAX_DATA is supposed to have the number of descriptor X the mmc
block size and desc_count contain all this information + 1.

Reported by: phk

r358635:
dwmmc: Rework the DMA engine

Each segment can be up to 4096 bytes in chain structure according to the
RK3399 TRM Part 2.
Set the buffers in full ring where the last one point to the first one.
Correctly reports the MMC_IVAR_MAX_DATA.
Use CACHE_LINE_SIZE for bus_dma alignment.