devfreq: Suspend all devices on system shutdown

devfreq: Suspend all devices on system shutdown

This way devfreq core ensures that all its devices will be set to safe
operation points before reboot operation. There are board on which some
aggressive power saving operation points are behind the capabilities of
the bootloader to properly reset the hardware and boot the board. This
way one can avoid board crash early after reboot.

Similar pattern is used in CPUfreq subsystem.

Reported-by: Markus Reichl <m.reichl@fivetechno.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I45467c8e02f7fedf90d1dfd5080e6143a8cb69f7