ARM: dma-mapping: fix nommu dma_alloc_coherent()

The nommu version of 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>