hdl: add FPGA based digital loopback RX mux

This commit introduces an RX mux mode where TX samples that are about to be
written to the LMS are piped into the RX sample FIFO. RX and TX must be running
at the sample sample rate. RX was not designed to latch on any specific event so
the first TX sample is dropped by RX FIFO. Software utilities looking to test
the behavior of the RX FIFO should anticipate that behavior as normal.
hdl: add LED control

Setting configuration GPIO bit 15 to 1 enables control over the LEDs.
  Bit 15: Enable LED control
  Bit 14: LED3 value
  Bit 13: LED2 value
  Bit 12: LED1 value

To disable all LEDs in bladeRF-cli run `set gpio 0x8057'.
To enable all LEDs in bladeRF-cli run `set gpio 0xf057'.