uboot-imx

usb: dwc2: Implement SPLIT transaction support

In contrast to non-SPLIT transfers each transaction has to be submitted
as an individual chunk.
The transaction state machine proceeds from SSPLIT to CSPLIT if the ACK
flag is set. CSPLIT has to be repeated while NYET is set.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>