powerpc/85xx: disable timebase synchronization under the hypervisor

The Freescale hypervisor does not allow guests to write to the timebase
registers (virtualizing the timebase register was deemed too complicated),
so don't try to synchronize the timebase registers when we're running
under the hypervisor.

This typically happens when kexec support is enabled.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>