coresight: add driver support for scatter gather feature

Add driver support to enable scatter-gather feature with TMC configuration.
Scatter gather option for ETR DDR mode supports the use of a table of
locations of 4K blocks of physical memory. Since this memory is not
necessarily contiguous, user can increase the trace buffer size as long as
DDR memory is available.

Default memory option is "contig" for TMC ETR DDR configuration. User can
enable scatter gather feature dynamically.

Change-Id: I74136fb86fe751aa33607d3922dce5d572f0a4a2
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>