edma: Merge EDMA private API headers with DMA engine

In preparation for moving EDMA private users to DMA Engine, lets merge
the headers and have everyone just include one header. We merge the
include/linux/platform_data/edma.h -> include/linux/edma.h and convert
all include directives to point include/linux/edma.h.

Regardless of non-DT/DT platform, everyone can include just this one
header to get the platform_data declarations.

Also this avoids confusion as to what should go into either header.

Lastly, davinci_mmc has been converted to use the DMAengine, drop the platform_data
header include which is no longer needed. All edma declarations are in linux/edma.h
which is included.

Cc: Matt Porter <matt@ohporter.com>
Signed-off-by: Joel A Fernandes <joelagnel@ti.com>