Move EDMA platform device creation into SoC-specific init code,
and provide it with platform_data to parameterize some of the
key differences.

Handle some key dm646x differences.  Ones that can change the
behavior including supporting all the DMA slots (512 vs just 128)
and a different set of no-events channels.

This doesn't yet ioremap(), or support using more than the first
two TCs on platforms like dm646x or dm365.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>