Platform/SiFive: add VariablePolicy resolutions

Add resolutions for
- SafeIntLib
- VariablePolicyLib
- VariablePolicyHelperLib
- VariablePolicyLib (DXE_RUNTIME_DRIVER)

for u500/u540 platforms.

Cc: Abner Chang <abner.chang@hpe.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>