Device tree idle state bindings changed in kernel v3.18. This patch
updates the FVP DT files to use PSCI suspend as idle state.

The patch also updates the 'compatible' property in the PSCI node
and the 'entry-method' property in the idle-states node in the FVP
Foundation GICv2-legacy device tree.

Change-Id: Ie921d497c579f425c03d482f9d7b90e166106e2f