Platform/SiFive: Add VariablePolicyHelperLib to the dsc files

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3489



Fix build error of U540 and U500 caused by BdsDxe.
The edk2 patch
 MdeModulePkg/BdsDxe: Update BdsEntry to use Variable Policy
requires VariablePolicyHelperLib, but U500.dsc and U540.dsc
didn't have it in the LibraryClasses section.

Cc: Abner Chang <abner.chang@hpe.com>
Cc: Gilbert Chen <gilbert.chen@hpe.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Signed-off-by: Takuto Naito <naitaku@gmail.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>