Add PC mux in cv32e40x_controller_fsm. use this mux for fencei PC and exception PC

Created by: silabs-oivind

PR consists of 3 commits: ccd2871f: SEC clean. Move PC mux from CS registers to controller FSM f7432030: Not SEC clean. Update the halt/kill behavior upon fencei. This is not SEC clean because the previous implementation would try to fetch an (unneeded) instruction when IF was killed when fencei entered WB. Halting IF (not killing) prevents this redundant instruction fetch. 5069951c: SEC clean. Use output from PC mux in controller FSM upon fencei.