[XEN][POWERPC] Allow Xen to use RTAS if available

[XEN][POWERPC] Allow Xen to use RTAS if available
If FW provides an RTAS layer, it will be instantiated and Xen will try
to use it for power control (halt, reboot, power-off, etc).  Xen will
also communicate to Dom0 (using /xen/power-control in the devtree)
when Xen cannot control power and it is hoped that the domain can,
especially useful for Maple boards running non-RTAS versions of PIBS.
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>