ARM: fix IXP4xx build failure

arm: export dma_set_coherent_mask

While a regression was fixed with commit 710224fa (arm: fix
"arm: fix pci_set_consistent_dma_mask for dmabounce devices"), a
new one was introduced as dma_set_coherent_mask wasn't exported
for modules. This patch takes care for this issue.

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>