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This patch introduces a power down handler which can be overriden
by SoCs to customise the power down process. The current SoCs do
not have a way of powering down the entire system as external PMIC
chips are involved in the process.

But future SoCs will have a way to power off the entire system
without talking to an external PMIC.

Change-Id: Ie7750714141a29cb0a1a616fafc531c4f11d0985
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>