This patch streamlines the differences between the MX23 and MX28 by
implementing a few helper functions to handle different DMA channel
mapping, different clock domain for SSP block and fixes a few minor
bugs.

First of all, the DMA channel mapping is now fixed in dma.h by defining
the actual channel map for both MX23 and MX28. Thus, MX23 now does no
longer use MX28 channel map which was wrong. Also, there is a fix for
MX28 DMA channel map, where the last four channels were incorrect.

Next, because correct DMA channel map is in place, the mxs_dma_init_channel()
call now bases the channel ID starting from SSP port #0. This removes the
need for DMA channel offset being added and cleans up the code. For the
same reason, the SSP0 offset can now be used in mxs_dma_desc_append(), thus
no need to adjust dma channel number in the driver either.

Lastly, the SSP clock ID is now retrieved by calling mxs_ssp_clock_by_bus()
which handles the fact that MX23 has shared SSP clock for both ports, while
MX28 has per-port SSP clock.

Finally, the mxs_ssp_bus_id_valid() pulls out two implementations of the
same functionality from MMC and SPI driver into common code.
Signed-off-by: NMarek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>