[keymgr_dpe, rtl] Raise error for Out-Of-Bound src / dst index

Description

During the keymgr_dpe integration into earlgrey the register interface was slightly changed. The keymgr_dpe now supports a NumMaxHwSlot and a NumInstHwSlot. Each top can set the number of instantiated hw slots while all input registers can account for NumMaxHwSlot hw slots. Currently a assign verifies that the src / dst index is smaller than NumInstHwSlot. In the future this will raise a invalid_op error (not yet implemented).

During randomize chip_csr_rw testing this assertion can be triggered when the DV writes a invalid slot number into the control register. As intermediate solution the writes into this register are restricted during randomized testing.