synclink_gt: add rx DMA buffer fill level control

Add run time control for receive DMA buffer fill level to allow
application to control receive latency when using stream oriented serial
protocols that pass receive data to application only after a DMA buffer
fills.  This was previously a compile time option, but run time control is
needed when application changes data rate (and latency requirements) or
uses different data rates on different ports.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>