Platform/Sgi: Add VariableFlashInfoLib to fix missing dependency

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3479

A recent change in MdeModulePkg [1] introduced VariableFlashInfoLib as a
dependency to support dynamic variable flash information. Add an
instance for the library class VariableFlashInfoLib in
SgiPlatformMm.dsc.inc to resolve this dependency.

[1]: https://github.com/tianocore/edk2/commit/8db39c60cdf35e0a53ccdbccf7e152ab41f54f4c



Signed-off-by: Prabin CA <prabin.ca@arm.com>