armv7m_trace_itm_config: wait for ITMBusy to be cleared

armv7m_trace_itm_config: wait for ITMBusy to be cleared

pg315 of CoreSight Components:

It is recommended that the ITMEn bit is cleared and waits for the
ITMBusy bit to be cleared, before changing any fields in the
Control Register, otherwise the behavior can be unpredictable.

Change-Id: Ie9a2b842825c98ee5edc9a35776320c668047769
Signed-off-by: Adrian Negreanu <adrian.negreanu@nxp.com>
Reviewed-on: http://openocd.zylin.com/6043
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>