MdeModulePkg: Use LockBoxNullLib for RISC-V

MdeModulePkg: Use LockBoxNullLib for RISC-V

RISC-V doesn't have SMM.

BZ:2562:
https://bugzilla.tianocore.org/show_bug.cgi?id=2562

Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Gilbert Chen <gilbert.chen@hpe.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
# Conflicts:
#	MdeModulePkg/MdeModulePkg.dsc