arm64: Implement custom mmap functions for dma mapping

The current dma_ops do not specify an mmap function so maping
falls back to the default implementation. There are at least
two issues with using the default implementation:

1) The pgprot is always pgprot_noncached (strongly ordered)
memory even with coherent operations
2) dma_common_mmap calls virt_to_page on the remapped non-coherent
address which leads to invalid memory being mapped.

Fix both these issue by implementing a custom mmap function which
correctly accounts for remapped addresses and sets vm_pg_prot
appropriately.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
[catalin.marinas@arm.com: replaced "arm64_" with "__" prefix for consistency]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>