ARM: dma-mapping: simplify page_to_dma() and __pfn_to_bus()

The non-highmem() and the __pfn_to_bus() based page_to_dma() both
compile to the same code, so its pointless having these two different
approaches.  Use the __pfn_to_bus() based version.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-By: Jamie Iles <jamie@jamieiles.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>