soc/tegra: pmc: Use sys-off handler API to power off Nexus 7 properly

Nexus 7 Android tablet can be turned off using a special bootloader
command which is conveyed to bootloader by putting magic value into the
special scratch register and then rebooting normally. This power-off
method should be invoked if USB cable is connected. Bootloader then will
display battery status and power off the device. This behaviour is
borrowed from downstream kernel and matches user expectations, otherwise
it looks like device got hung during power-off and it may wake up on
USB disconnect.

Switch PMC driver to sys-off handler API, which provides drivers with
chained power-off callbacks functionality that is required for powering-off
devices properly. It also brings resource-managed API for the restart
handler registration that makes PMC driver code cleaner.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>