Re-factor header files for easier PSCI library integration

Re-factor header files for easier PSCI library integration

This patch re-factors the following headers to make it easier to
integrate the PSCI library with an AArch32 Secure Payload :

  *  bl_common.h : The entry point information and the param
     header data structures are factored out into separate
     headers ep_info.h and param_headers.h
  *  psci.h : The PSCI library interfaces are factored out
     into the new header psci_lib.h
  *  context_mgmt.h : The header file is modified to not include
     arch.h when compiled for AArch32 mode.

No functional changes are introduced by this patch.

Change-Id: I5e21a843c0af2ba8e47dee4e577cf95929be8cd4
Signed-off-by: Soby Mathew <soby.mathew@arm.com>