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: I5d9f37b47c5bdd711161e7482ca3446549709406
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>