move all decode to execute signal registering to a seperate module

Currently, only the cappuccino pipeline implementation register
the signals from decode stage into execute stage,
(pronto)espresso does not have a seperate execute stage.

So, instead of trying to accomodate for both use cases in the
mor1kx_decode module, break out the registering of the signals
and all cappuccino specific logic into a seperate
mor1kx_decode_execute_cappuccino module.
If (or when) a new pipeline implementation arises that would
benefit from using this module, it should be determined whether
the _cappuccino in the name can be dropped (i.e., is it generic
enough to be used across pipeline implementations).

Also prefix all output signals coming from mor1kx_decode with
decode_, to make it clear what their origin is in the cpu_xxx
modules.
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.
cappuccino/execute_ctrl: omit reset logic to ctrl_rfb_o