ARM: etm: Support multiple ETMs/PTMs.

If more than one ETM or PTM are present, configure all of them
and enable the formatter in the ETB. This allows tracing on dual
core systems (e.g. omap4).

Change-Id: I028657d5cf2bee1b23f193d4387b607953b35888
Signed-off-by: Arve Hjønnevåg <arve@android.com>