ARM: 8286/1: mm: Fix dma_contiguous_reserve comment

DMA contiguous allocations have been able to use highmem since commit
"95b0e655 ARM: mm: don't limit default CMA region only to low memory"
but a comment still notes the earlier "low memory" limitation.  Update
the comment to remove the low memory limitation and fix the
s/contigouos/contiguous/ typo while we're at it.

Signed-off-by: George G. Davis <george_davis@mentor.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>