ARM: etm: Wait for etm/ptm(s) to stop before requesting PowerDown

When PowerDown was requested at the same time as ProgBit, the
formatter flush command that follows could get stuck.

Change-Id: Iafb665f61f055819e64ca1dcb60398c656f593e4
Signed-off-by: Arve Hjønnevåg <arve@android.com>