UefiPayloadPkg: Store the size of the MMCONF window

Store the real size of the Pcie Memory Mapped Address Space.
This change is necessary to support variable size of MMCONF spaces.

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: Maurice Ma <maurice.ma@intel.com>
Cc: Guo Dong <guo.dong@intel.com>
Cc: Benjamin You <benjamin.you@intel.com>
Reviewed-by: Guo Dong <guo.dong@intel.com>
Reviewed-by: Maurice Ma <maurice.ma@intel.com>