PM / shmobile: Make MTU2 driver use pm_genpd_dev_always_on()

Make the MTU2 clocksource driver mark its device as "always on"
using pm_genpd_dev_always_on() to protect it from surprise power
removals.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Simon Horman <horms@verge.net.au>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Cc: stable@vger.kernel.org