For PPS output (perout period is 1.000000000), accept the new "phase"
parameter from the periodic output request structure.

For both PPS and freeform output, accept the new "on" argument for
specifying the duty cycle of the generated signal. Preserve the old
defaults for this "on" time: 1 us for PPS, and half the period for
freeform output.

Also preserve the old behavior that accepted the "phase" via the "start"
argument.
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>