ARM: DMA coherent allocator: align remapped addresses

The DMA coherent remap area is used to provide an uncached mapping
of memory for coherency with DMA engines.  Currently, we look for
any free hole which our allocation will fit in with page alignment.

However, this can lead to fragmentation of the area, and allows small
allocations to cross L1 entry boundaries.  This is undesirable as we
want to move towards allocating sections of memory.

Align allocations according to the size, limiting the alignment between
the page and section sizes.

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