[POWERPC] Various fixes to pcibios_enable_device()

Our implementation of pcibios_enable_device() has a couple of problems.

One is that it should not check IORESOURCE_UNSET, as this might be
left dangling after resource assignment (shouldn't but there are
bugs), but instead, we make it check resource->parent which should
be a reliable indication that the resource has been successfully
claimed (it's in the resource tree).

Then, we also need to skip ROM resources that haven't been enabled
as x86 does.

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