feat(fdt): add the ability to supply idle state information

Some platforms require extra firmware to implement CPU_SUSPEND, or only
have working CPU_SUSPEND in certain configurations. On these platforms,
CPU idle states should only be listed in the devicetree when they are
actually available. Add a function BL31 can use to dynamically supply
this idle state information.

Change-Id: I64fcc288303faba8abec4f59efd13a04220d54dc
Signed-off-by: Samuel Holland <samuel@sholland.org>