juno: Add a handler for 'ARM Architecture' SMC calls ...

Currently UEFI and Linux are using SMC calls in the 'ARM Architecture'
Owning Entity range so lets implement these to get things working.

UEFI probably doesn't actually need to issue the ID_PRESENCE and ID_UID
calls it does, and the device-tree used by Linux could specify the PSCI
identifiers instead. After those changes, this patch isn't required.

Signed-off-by: Jon Medhurst <tixy@linaro.org>