usb: musb: Add support of CPPI 4.1 DMA controller to DA8xx

Currently, only the PIO mode is supported.
This add support of CPPI 4.1 to DA8xx.
As in DA8xx the CPPI 4.1 DMA is a part of the USBSS, create the CPPI 4.1
device as a child of USB.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
[b-liu@ti.com: minor tweak in the commit log]
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>