PCI: Use a weak symbol for the empty version of pcibios_add_platform_entries()

I'm not sure if this is going to fly, weak symbols work on the compilers I'm
using, but whether they work for all of the affected architectures I can't say.
I've cc'ed as many arch maintainers/lists as I could find.

But assuming they do, we can use a weak empty definition of
pcibios_add_platform_entries() to avoid having an empty definition on every
arch.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>