Newer firmware provides 4 programmable signal generators, add
support for those here.  The signal generators provide the
ability to set the period, duty cycle, phase offset, and polarity,
with new values defaulting to prior values.

The period and phase offset are specified in nanoseconds.

E.g:    period [duty [phase [polarity]]]

  echo 500000000 > signal	# 1/2 second period
  echo 1000000 40 100 > signal	# 1ms period, 40% on, offset 100ns
  echo 0 > signal		# turn off generator
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>