PM / devfreq: exynos-bus: Suspend all devices on system shutdown

Force all Exynos buses to safe operation points before doing the system
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.

This fixes reboot issue on OdroidU3 board both with eMMC and SD boot.

Reported-by: Markus Reichl <m.reichl@fivetechno.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>