MIPS: Malta: support powering down

This patch powers down the Malta in response to a power off command (eg.
poweroff or shutdown -P). It may then be powered back up by pressing the
"ON/NMI" button (S4) on the board. In cases where the power off state
cannot be entered (eg. because the required PCI support is disabled) the
current reset behaviour will be used as a fallback.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Tested-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6907/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>