[POWERPC] Resolve the BUID for RTAS PCI config space accesses

The BUID is the first entry of a PCI host bridge "reg" property.

Now that PCI busses can be anywhere in the device-tree, we need to
fully translate the value there to a CPU physical address before
we can use it with RTAS.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>