ARM: dma-mapping: simplify dma_cache_maint_page

dma_cache_maint_contiguous is now simple enough to live inside
dma_cache_maint_page, so move it there.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-By: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>