1. Add defines for MAX values for UEFI data types.

1. Add defines for MAX values for UEFI data types.
2. Remove the #defines and add the extern declarations for gEfiDebugPortVariableGuid and gEfiDebugPortDevicePathGuid.

Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: michael.d.kinney@intel.com
Reviewed-by: Star Zeng <star.zeng@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14465 6f19259b-4bc3-4df7-8a09-765794883524