ARM: xen: only set pm function ptrs for Xen guests

commit 9dd4b2944c46e1fdbd0a516c221c8a2670cbf005 upstream.

xen_pm_init was unconditionally setting pm_power_off and arm_pm_restart
function pointers. This breaks multi-platform kernels. Make this
conditional on running as a Xen guest and make it a late_initcall to
ensure it is setup after platform code for Dom0.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>