ARM: firmware: add prepare_idle() operation

Some firmwares do not put the CPU into idle mode themselves, but still
need to be informed that the CPU is about to enter idle mode before this
happens. Add a prepare_idle() operation to the firmware_ops structure to
handle such cases.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Stephen Warren <swarren@nvidia.com>