SecurityPkg/PlatformSecureLibNull: permit use by MM_STANDALONE modules

SecurityPkg/PlatformSecureLibNull: permit use by MM_STANDALONE modules

Add MM_STANDALONE to the list of module types that are permitted to
link to this library. Also, since the constructor prototype is
different between MM_STANDALONE and DXE_DRIVER type libraries,
convert the library into BASE type.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>