coresight: tmc: add support to configure etr memory size

Add support to expose a node for user to configure memory buffer size
for an ETR device.

Change-Id: I18e8688c45d87b26a1aaac25a91b2deaaf61272e
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>