mor1kx-defines: fix typo in OR1K_ALU_OPC_SECONDARY_SELECT
decode: break down immediate decoding into seperate signals
decode: remove redundant mux for decode_opc_alu_o

The adder will anyways produce the right result without this.
cappuccino: remove op_alu control signal

op_alu is only used to generate the execute_waiting_o signal,
by anding it to alu_valid, but alu_valid will only be
deasserted on instructions that can stall the alu, so it's
not needed to also check op_alu.