hikey960: Enable system power off callback

hikey960: Enable system power off callback

On Hikey960 if outputs GPIO176 low level, it can tell PMIC to power off
the whole board.  To avoid resetting the board and stay off, it also
requires the SW2201's three switches 1/2/3 need to be all set to 0.

Since current code doesn't contain complete GPIO modules and misses to
support GPIO176.  This patch adds all known GPIO modules and initialize
GPIO in BL31, and adds system power off callback to use GPIO176 for PMIC
power off operation.

Change-Id: Ia88859b8b7c87c061420ef75f0de3e2768667bb0
Signed-off-by: Leo Yan <leo.yan@linaro.org>