x86/PCI: MMCONFIG: centralize MCFG structure management

This patch encapsulate pci_mmcfg_config[] updates.  All alloc/free is now
done in pci_mmconfig_add() and free_all_mcfg(), so all updates to
pci_mmcfg_config[] and pci_mmcfg_config_num are in those two functions.

This replaces the previous sequence of extend_mmcfg(), fill_one_mmcfg()
with the single pci_mmconfig_add() interface.  This interface is currently
static but will eventually be used in the host bridge hot-add path.

Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>