[ARM] 3500/1: fix PXA27x DMA allocation priority

Patch from Nicolas Pitre

Intel PXA27x developers manual section 5.4.1.1 lists a priority
distribution for the DMA channels differently than what the code
currently assumes.  This patch fixes that.

Noticed by Simon Vogl <vogl@soft.uni-linz.ac.at>

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>