target-arm: add emulation of PSCI calls for system emulation

Add support for handling PSCI calls in system emulation. Both version
0.1 and 0.2 of the PSCI spec are supported. Platforms can enable support
by setting the "psci-conduit" QOM property on the cpus to SMC or HVC
emulation and having a PSCI binding in their dtb.

Signed-off-by: Rob Herring <rob.herring@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1412865028-17725-7-git-send-email-peter.maydell@linaro.org
[PMM: made system reset/off PSCI functions power down the CPU so
 we obey the PSCI API requirement never to return from them;
 rearranged how the code is plumbed into the exception system,
 so that we split "is this a valid call?" from "do the call"]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
