powerpc/powernv/pci: Add helper to find ioda_pe from BDFN

For each PHB we maintain a reverse-map that can be used to find the
PE that a BDFN is currently mapped to. Add a helper for doing this
lookup so we can check if a PE has been configured without looking
at pdn->pe_number.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200417073508.30356-2-oohall@gmail.com