feat(plat/mdeiatek/mt8186): add power-off function for PSCI

Add support for system-off.

TEST=build pass
BUG=b:202871018

Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Change-Id: Ic734696aab1b71ae85bca6ed08e544a522ce5c95
1 file changed