dmaengine: cppi41: rename platform variables

Currently, only the AM335x is supported by the driver.
Though the driver has a glue layer to support different platforms,
some platform variable names are not prefixed with the platform name.
To facilitate the addition of a new platform,
rename some variables owned by the AM335x glue.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>