Explain the two flow control methods which the PL08x implements, along
with the problem which peripheral flow control presents.  This helps
people understand why we are unable to use these DMA controllers with
(eg) the MMCI.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Acked-by: NLinus Walleij <linus.walleij@stericsson.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>