Adds capability to independently delay the Auto T/R switching signal

Adds capability to independently delay the Auto T/R switching signal
by a configurable number of clock ticks, to allow users to precisely
align their T/R output with the pipeline delays in the transmitter.

There are two new registers:

FR_ATR_TX_DELAY (7'd2)
FR_ATR_RX_DELAY (7'd3)

...and the corresponding db_base.py methods to set them:

db_base.set_atr_tx_delay(clock_ticks)
db_base.set_atr_rx_delay(clock_ticks)

These methods are inherited by all the daughterboard objects so you can
call them from your scripts as:

subdev.set_atr_tx_delay(...)

...where 'subdev' represents the daughtercard object you're working with.

The FPGA synthesis for the 2 RXHB, 2 TX case expands from 95% to 96%,
with no additional synthesis messages or impact on timing.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@5022 221aa14e-8319-0410-a670-987f0aec2ac5