This patch implements the necessary topology changes for supporting
system power domain on CSS platforms. The definition of PLAT_MAX_PWR_LVL and
PLAT_NUM_PWR_DOMAINS macros are removed from arm_def.h and are made platform
specific. In addition, the `arm_power_domain_tree_desc[]` and
`arm_pm_idle_states[]` are modified to support the system power domain
at level 2. With this patch, even though the power management operations
involving the system power domain will not return any error, the platform
layer will silently ignore any operations to the power domain. The actual
power management support for the system power domain will be added later.

Change-Id: I791867eded5156754fe898f9cdc6bba361e5a379

This patch migrates ARM reference platforms, Juno and FVP, to the new platform
API mandated by the new PSCI power domain topology and composite power state
frameworks. The platform specific makefiles now exports the build flag
ENABLE_PLAT_COMPAT=0 to disable the platform compatibility layer.

Change-Id: I3040ed7cce446fc66facaee9c67cb54a8cd7ca29