This patch adds support for shutdown/reboot handlers to the MCE
driver.

ATF communicates with mce using nvg interface for shutdown &
reboot. Both shutdown and reboot use the same nvg index.
However, the 1st bit of the nvg data argument differentiates
whether its a shutdown or reboot.

Change-Id: Id2d1b0c4fec55abf69b7f8adb65ca70bfa920e73
Signed-off-by: Vignesh Radhakrishnan <vigneshr@nvidia.com>