coresight: use dma_alloc_coherent for allocating tmc-etr memory

The CoreSight TMC driver when configured for ETR requires uncached
memory in RAM to which it routes trace data. In order to allocate
this memory replace existing allocate_contiguous_ebi api with
dma_alloc_coherent api which is the linux standard of allocating
memory.

Change-Id: I59f88009f2abed95fd9b81ea92a7d484b9d6b833
Signed-off-by: Aparna Das <adas@codeaurora.org>