GTP reference clock is connected to CLK01 and/or CLK11 in our instance of GTP_DUAL.

According to UG386, figure 2-3, page 41, REFSELDYPLLx must be set to '4' to select
CLK10/CLK11.

This is done in order to ensure backward compatibility with previous versions of
the Xilinx platform support package.

Otherwise, counter gets crazy when watchdog resets the swcore and feeds all the
endpoints and NIC with ack=1.

This way one Tx OOB word may be lost, and tx timestamp won't be generated
causing _wrn_start_xmit: descriptor overflow: tx timestamp pending_ message from
wr-nic driver

- removed declarations of unused signals
- reduced padding_size to bits needed for padding possible runt Ethernet
  frames
- simplified signal assignments and conditions in FSM (e.g. no need to
  check if needs_padding = 1 and padding_size /= 0, because when former
  is true, the latter is always true too)
- moved assignment of outputs validated with other outputs outside FSM
  to simplify the logic (fake rtu decision)

Instead of old fabtic converted to Pipelined WB using ep_rx_wb_master
module from endpoint_private_pkg

- removed ARB_DISABLED state, instead FSM is hold in ARB_START_SCAN
  unless module is enabled
- removed declarations of unused signals (granted_desc_idx, desc_idx_d0,
  check_count, stupid_hack)
- moved assignment of outputs validated with other signals outside FSM
  to simplify the logic (txdesc_current_o/rxdesc_current_o are validated
  by desc_grant_o, and stored in internal registers of nic_rx_fsm and
  nic_tx_fsm)
- rewritten ARB_WRITE_DESC state, so that data output is driven by
  desc_subreg counter instead of separate counter in case statement.
- more exotic regnums in marshalling functions (nic_descriptors_pkg) to
  minimize logic required to output correct data when needed in modified
  ARB_WRITE_DESC state