ARM: dma-mapping: fix coherent arch dma_alloc_coherent()

The coherent architecture dma_alloc_coherent was using kmalloc/kfree to
manage the memory.  dma_alloc_coherent() is expected to work with a
granularity of a page, so this is wrong.  Fix it by using the helper
functions now provided.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Greg Ungerer <gerg@uclinux.org>