MdePkg: PciExpressLib support variable size MMCONF

Add support for arbitrary sized MMCONF by introducing a new PCD.
Add a return value to point out invalid PCI addresses.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Marcello Sylvester Bauer <marcello.bauer@9elements.com>
Cc: Patrick Rudolph <patrick.rudolph@9elements.com>
Cc: Christian Walter <christian.walter@9elements.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>