XIF - Fix decoder_ctrl_t

Created by: Silabs-ArjanB

• Change

logic [REGFILE_NUM_READ_PORTS-1:0] rf_re;

into

logic [1:0] rf_re; // Decoder only supports rs1, rs2

• Fix https://github.com/openhwgroup/cv32e40x/blob/a468b8fbc068a80e7496b6235b50c249e0e92c34/rtl/cv32e40x_id_stage.sv#L429 to 0 extend rf_re. rf_re needs to be fixed width 2; rf_re_o needs to remain parameterized width. Also add comment that internal (i.e. non-offloaded) instructions use at most 2 read ports.

• Remove parameter REGFILE_NUM_READ_PORTS = 2; from package file and make it a top level localparam that is set to 2 if X_EXT = 0 and to X_NUM_RS otherwise. Feed REGFILE_NUM_READ_PORTS parameter to where it is needed

• Add following parameters with defaults as suggested i nhttps://cv32e40x-user-manual.readthedocs.io/en/latest/integration.html

  .X_EXT (will be further hooked up by Michael)
  .X_NUM_RS (use this one)
  .X_ID_WIDTH (ignore for now)
  .X_MEM_WIDTH (ignore for now)
  .X_RFR_WIDTH (ignore for now)
  .X_RFW_WIDTH (ignore for now)
  .X_MISA (ignore for now)