arm: parse PSCI node from the host device-tree

arm: parse PSCI node from the host device-tree

The availability of a PSCI handler is advertised in the DTB.
Find and parse the node (described in the Linux device-tree binding)
and save the function number for bringing up a CPU for later usage.
We do some sanity checks, especially we deny using HVC as a calling
method, as it does not make much sense currently under Xen.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>