clocksource: sh_cmt: wait for CMCNT on init V2

Add code to the CMT driver to wait for CMCNT V2. This to let
the register value settle before starting the timer channel.
Makes the driver more robust.

Needed for CMT2 on sh7372 and certain CMT channels on sh73a0.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>