dmaengine: add dma_request_slave_channel_compat()

Adds a dma_request_slave_channel_compat() wrapper which accepts
both the arguments from dma_request_channel() and
dma_request_slave_channel(). Based on whether the driver is
instantiated via DT, the appropriate channel request call will be
made.

This allows for a much cleaner migration of drivers to the
dmaengine DT API as platforms continue to be mixed between those
that boot using DT and those that do not.

Suggested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Matt Porter <mporter@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>