The platform code generates the power domain tree. The handler to
retrieve the tree should also reside in the platform code.

This patch moves the plat_get_power_domain_tree_desc() to the
individual platforms.

Change-Id: Iaafc83ed381d83129501111ef655e3c58a8a553f
Signed-off-by: Anthony Zhou <anzhou@nvidia.com>
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>