ARM: fix regression in IXP4xx clocksource

Commit 234b6ced

   clocksource: convert ARM 32-bit up counting clocksources

broke the build for ixp4xx and made big endian operation impossible.
This commit restores the original behaviour.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>
[ Thomas says that we might want to have generic BE accessor functions
  to the MMIO clock source, but that hasn't happened yet, so in the
  meantime this seems to be the short-term fix for the particular
  problem - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>