sh: MTU2: Introduce clock events suspend/resume routines

Introduce suspend/resume routines for SH MTU2 clock event devices
such that if those devices belong to a PM domain, the generic PM
domains framework will be notified that the given domain may be
turned off (during system suspend) or that it has to be turned on
(during system resume).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Magnus Damm <damm@opensource.se>