The capabilities exposed by the PSCI generic layer depends on the hooks
populated by the platform in `plat_arm_psci_pm_ops`. Currently ARM
Standard platforms statically define this structure. However, some
platforms may want to modify the hooks at runtime before registering
them with the generic layer.

This patch introduces a new ARM platform layer API
`plat_arm_psci_override_pm_ops` which allows the platform to probe
the power controller and modify `plat_arm_psci_pm_ops` if required.
Consequently, 'plat_arm_psci_pm_ops' is no longer qualified as
`const` on ARM Standard platforms.

Change-Id: I7dbb44b7bd36c20ec14ded5ee45a96816ca2ab9d
Signed-off-by: Soby Mathew <soby.mathew@arm.com>