ARM: EXYNOS: move restart code into pmu driver

Let's register restart handler from PMU driver for restart
functionality. So that we can remove restart hooks from
machine specific file, and thus moving ahead when PMU moved
to driver folder, this functionality can be reused for ARM64
based Exynos SoC's.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>