coresight: add device attribute for etr ddr mem type

Add device attribute for memory type option with TMC ETR DDR configuration.
This is in preparation for scatter gather feature with TMC ETR
configuration.

Change-Id: If72078f89442e9c58e09392fbced362126dd15e7
Signed-off-by: Sarang Joshi <spjoshi@codeaurora.org>