xtensa: drop platform_halt and platform_power_off

Instead of using xtensa-specific platform_halt and platform_power_off
callbacks use do_kernel_power_off in the machine_halt and
machine_power_off and reimplement existing platform_halt and
platform_power_off users with register_sys_off_handler.

Drop platform_halt and platform_power_off declarations and default
implementations.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>