ANDROID: dma-contiguous: Add tracehook to allow subpage allocations in dma_alloc_contiguous

Add a tracehook to allow callers into dma_alloc_contiguous() to make
use of the built-in CMA area if the caller has addressing limitations;
this provides a means of allocating from memory whose bounds are
restricted to the lower 4 GB of memory, without having to enable DMA32
(assuming the default CMA area has been restricted to the appropriate
address ranges).

Leaf changes summary: 1 artifact changed
Changed leaf types summary: 0 leaf type changed
Removed/Changed/Added functions summary: 0 Removed, 0 Changed, 0 Added function
Removed/Changed/Added variables summary: 0 Removed, 0 Changed, 1 Added variable

1 Added variable:

  [A] 'tracepoint __tracepoint_android_vh_subpage_dma_contig_alloc'

Bug: 199917449
Change-Id: Ia86fb416376bca231405b06ab27b0674c8fe3e14
Signed-off-by: Chris Goldsworthy <quic_cgoldswo@quicinc.com>