Add subclass of simplebus for Broadcom XLP

This will override the resource allocation of simplebus, and also
merge the resource allocation code which was in xlp_pci.c.

With this change the SoC devices that does not have proper PCI
resources will be on the FDT simplebus. We can remove
sys/mips/nlm/dev/cfi_pci_xlp.c and sys/mips/nlm/dev/uart_pci_xlp.c