Add a default do-nothing implementation of fdt_pci_devmap() using a weak alias,

Add a default do-nothing implementation of fdt_pci_devmap() using a weak alias,
so that we don't need an empty implementation of it for every Marvell platform
that has no PCI.  This allows the removal of the SheevaPlug-specific stub and
config files, and eliminates the need to add similar stubs for future models.

Marvell platforms that do expose PCI are compiled with 'device pci' which
causes the real (non-weak) implementation in dev/fdt/fdt_pci.c to be used.

Approved by:	cognet (mentor)