Add special function that executes a specially crafted circular
DMA descriptor. The function doesn't wait for the descriptor to
finish the transfer, since the descritor never finishes. This is
useful when operating a SmartLCD through the LCDIF interface, as
the LCDIF does not give us any means to have continuous refresh
of the SmartLCD. Instead, the RUN bit in the LCDIF CTRL register
must be triggered manually. This can be worked around by starting
an DMA transfer which continuously sets the RUN bit. This function
allows starting exactly such transfer.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>