UefiPayloadPkg: Support variable size MMCONF space

The default size is still 256MiB, but will be overwritten by
UefiPayloadPkg with the real MMCONF size.

e.g.: On embedded AMD platforms the MMCONF window size is usually
      only 64MiB.

Fixes crash on platforms not exposing 256 buses.
Tested on:
* AMD Stoney Ridge

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: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Benjamin You <benjamin.you@intel.com>
Reviewed-by: Guo Dong <guo.dong@intel.com>