clocksource: sh_tmu: compute mult and shift before registration

Since commit 98962465 ("nohz: Prevent
clocksource wrapping during idle"), the CPU of an R2D board never goes
to idle. This commit assumes that mult and shift are assigned before
the clocksource is registered. As a consequence the safe maximum sleep
time is negative and the CPU never goes into idle.

This patch fixes the problem by moving mult and shift initialization
from sh_tmu_clocksource_enable() to sh_tmu_register_clocksource().

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Cc: stable@kernel.org
Signed-off-by: Paul Mundt <lethal@linux-sh.org>