Reorganise PSCI PM handler setup on ARM Standard platforms

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
diff --git a/plat/arm/css/common/aarch64/css_helpers.S b/plat/arm/css/common/aarch64/css_helpers.S
index 05bd864..1137a49 100644
--- a/plat/arm/css/common/aarch64/css_helpers.S
+++ b/plat/arm/css/common/aarch64/css_helpers.S
@@ -67,7 +67,7 @@
 	 * ---------------------------------------------------------------------
 	 */
 func plat_get_my_entrypoint
-	mov_imm	x0, TRUSTED_MAILBOX_BASE
+	mov_imm	x0, PLAT_ARM_TRUSTED_MAILBOX_BASE
 	ldr	x0, [x0]
 	ret
 endfunc plat_get_my_entrypoint