coresight: tmc: add scatter-gather support for ETR device

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: I3ce654392d2b75d24f7982638e53c2aab27d4a0e
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>