target: add profiling interface

Profiling could be target-specific.  Add .profiling interface
to target_type.

Change-Id: Ic0eea9db742971db1350a474fbbb5ed24565922b
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1571


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>