coresight: enable scatter gather feature based on dt entry

Based on sg-enable DT entry, enable scatter gather feature
for TMC ETR configuration. Scatter gather feature is not supported
on all targets. sg-enable entry will help to enable this feature
only on those targets which supports this feature.

Change-Id: I66fe50c653ad9646104b9dc0d77a0fa6f82503dd
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>