Revert "PM: domains: Fix up terminology with parent/child"

This reverts commit 35a2681e.

It breaks the Android kernel abi, so revert it.  If it needs to come
back later, it can do so in an abi-safe way.

Bug: 161946584
Change-Id: Iac9428a04b1edda4ce45b8186a99f68671bb7fca
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>