CLP retention cells checks

• A retention cell is required, but it is neither present in the design nor specified in the UPF file.
• A retention cell is not required (as the power domain is an always-on domain), but it is either present in the design or specified in the UPF file.
• A retention cell is present in the design, but it does not have the corresponding strategy defined in the UPF file.
• A retention strategy is specified in the UPF file, but a retention cell is not present in the design.
• The retention condition is not asserted when the power domain is powered-down.
• The retention control signal is not generated from an always-on domain, or is unconnected, or is driven by a constant.
• The retention condition is disabled at power down or power up.
• The retention condition toggles at power down.