The CPPI 4.1 DMA in USB subsystem shares its clock with the
USB OTG, and most of the time, the clock will be enabled by
USB.  But during the init of the DMA, USB is not enabled
(waiting for DMA), and then we must enable the DMA clock
before doing anything.

Add clock for the CPPI 4.1 DMA engine.
Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
[nsekhar@ti.com: minor commit message tweaks]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>