powerpc: Ultravisor: Fix the dependencies for CONFIG_PPC_UV

Let PPC_UV depend only on DEVICE_PRIVATE which in turn
will satisfy all the other required dependencies

Fixes: 013a53f2 ("powerpc: Ultravisor: Add PPC_UV config option")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200109092047.24043-1-bharata@linux.ibm.com