sh: turn off irqs when disabling CMT/TMU timers

Modify the CMT and TMU drivers to disable interrupts when
disabling the timer. Only using start/stop bits is not
enough.

This fixes a bootup hang on Migo-R when the CMT is replaced
by TMU for clockevents but the CMT keeps on delivering irqs
even though the timer start bit is off.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>