drivers: pm_cpu_ops: psci: provide sys_poweroff hook

Instead of implementing a custom power off API (pm_system_off),
implement the sys_poweroff hook, and indicate power off is supported by
selecting HAS_POWEROFF. Note that according to the PSCI specification
(DEN0022E), the SYSTEM_OFF operation does not return, however, an error
is printed and system is halted in case this occurs.

Note that the pm_system_off has also been deleted, from now on, systems
supporting PSCI should enable CONFIG_POWEROFF and call the standard
sys_poweroff() API.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>