WhitleySiliconPkg: Use same variable name for FspNvsHob.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3678



To simplify the implementation the variable Name/GUID has been
changed to "FspNvsBuffer" and gFspNvsBufferVariableGuid
regardless it stores the data from FSP_NON_VOLATILE_STORAGE_HOB2
or FSP_NON_VOLATILE_STORAGE_HOB.

Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>