This patch does the following reorganization to psci power management (PM)
handler setup for ARM standard platform ports :
1. The mailbox programming required during `plat_setup_psci_ops()` is identical
for all ARM platforms. Hence the implementation of this API is now moved
to the common `arm_pm.c` file. Each ARM platform now must define the
PLAT_ARM_TRUSTED_MAILBOX_BASE macro, which in current platforms is the same
as ARM_SHARED_RAM_BASE.
2. The PSCI PM handler callback structure, `plat_psci_ops`, must now be
exported via `plat_arm_psci_pm_ops`. This allows the common implementation
of `plat_setup_psci_ops()` to return a platform specific `plat_psci_ops`.
In the case of CSS platforms, a default weak implementation of the same is
provided in `css_pm.c` which can be overridden by each CSS platform.
3. For CSS platforms, the PSCI PM handlers defined in `css_pm.c` are now
made library functions and a new header file `css_pm.h` is added to export
these generic PM handlers. This allows the platform to reuse the
adequate CSS PM handlers and redefine others which need to be customized
when overriding the default `plat_arm_psci_pm_ops` in `css_pm.c`.
Change-Id: I277910f609e023ee5d5ff0129a80ecfce4356ede
785fb92b8a