powerpc/powernv/npu: Add release_ownership hook

In order to make ATS work and translate addresses for arbitrary
LPID and PID, we need to program an NPU with LPID and allow PID wildcard
matching with a specific MSR mask.

This implements a helper to assign a GPU to LPAR and program the NPU
with a wildcard for PID and a helper to do clean-up. The helper takes
MSR (only DR/HV/PR/SF bits are allowed) to program them into NPU2 for
ATS checkout requests support.

This exports pnv_npu2_unmap_lpar_dev() as following patches will use it
from the VFIO driver.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>