serial: ifx6x60: Add modem power off function in the platform reboot process

This patch add modem power off function in the reboot process according registering
reboot callback to the reboot_notifier_list. Also realizing the spi shutdown function.

Signed-off-by: Bi Chao <chao.bi@intel.com>
Signed-off-by: Chen Jun <jun.d.chen@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>