arm64: dma-mapping: make alloc_noncoherent more robust

Large allocations can result in the arm64_swiotlb_alloc_noncoherent
call not being able to succeed because it can't find enough
contiguous memory to setup the mapping.

Make arm64_swiotlb_alloc_noncoherent more robust by using vmalloc
as a fallback.

Change-Id: I00e8c3f634dc2280f3731c6042b9dd3dc644cb73
Signed-off-by: Liam Mark <lmark@codeaurora.org>