powerpc/83xx: Add shutdown request support to MCU handling on MPC8349 MITX

This patch add support for calling ctrl_alt_del() when the power button is
pressed for more than about 2 seconds on some freescale MPC83xx evaluation
boards and reference design.

The code uses a kthread to poll the CTRL_BTN bit each second.

Also change Kconfig entry of the driver to bool, as device's gpio
registration is broken when loading as module.

Tested on an MPC8315E RDB board.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>