clocksource: sh_cmt: compute mult and shift before registration

Based on the sh_tmu change in 66f49121
("clocksource: sh_tmu: compute mult and shift before registration").
The same issues impact the sh_cmt driver, so we take the same approach
here.

Cc: stable@kernel.org
Signed-off-by: Paul Mundt <lethal@linux-sh.org>