Juno: Support entry into a standby state

This patch adds support on the Juno platform for entering a wfi in
response to a PSCI CPU_SUSPEND call where the state type is a
standby state.

Change-Id: I0a102dee1f8d2ad936c63ad1d1d3ad001a4a4768
Merge pull request #216 from vikramkanigiri/vk/juno_standby_support

Juno: Support entry into a standby state