coresight: tmc: add scatter-gather support for tmc etr

Add support to configure ETR device in scatter-gather mode.
In scatter-gather mode trace buffer can be configured to use bigger
buffer size without need of bigger contiguous memory.

Change-Id: Ia15b9953758d8c3da74637f2e6b33480a51679e3
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>