openrisc: Use do_kernel_power_off()

After commit 14c56787 ("power: reset: syscon-poweroff: Use
devm_register_sys_off_handler(POWER_OFF)") setting up of pm_power_off
was removed from the driver, this causes OpenRISC platforms using
syscon-poweroff to no longer shutdown.

The kernel now supports chained power-off handlers. Use
do_kernel_power_off() that invokes chained power-off handlers.  All
architectures have moved away from using pm_power_off except OpenRISC.

This patch migrates openrisc to use do_kernel_power_off() instead of the
legacy pm_power_off().

Fixes: 14c56787 ("power: reset: syscon-poweroff: Use devm_register_sys_off_handler(POWER_OFF)")
Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>