coresight: use trigin 2 for cti reset trigger for tmc-etr

Flush ETR is mapped to TRIGIN[2] and so switch to using it in
order to cause a CTI trigger for TMC-ETR flush during wdog reset.

This avoids any race between TMC-ETR flush to memory and DDR
being put into self-refresh.

CRs-Fixed: 491986
Change-Id: Ia5e643121e040bd9c57c89a4b762b794bc8ddf62
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>