Add support for external synchronization clock via GPIOs.
1PPS signals are handled via the dedicated 3 GPIOs: SDP3_2,
SDP3_3 and GPIO_4.
Previously it was not possible to use the external PTP
synchronization clock.
All possible HW configurations are supported.
	SDP3_2,	SDP3_3,	GPIO_4
	off,	off,	off
	off,	in_A,	off
	off,	out_A,	off
	off,	in_B,	off
	off,	out_B,	off
	in_A,	off,	off
	in_A,	in_B,	off
	in_A,	out_B,	off
	out_A,	off,	off
	out_A,	in_B,	off
	in_B,	off,	off
	in_B,	in_A,	off
	in_B,	out_A,	off
	out_B,	off,	off
	out_B,	in_A,	off
	off,	off,	in_A
	off,	out_A,	in_A
	off,	in_B,	in_A
	off,	out_B,	in_A
	out_A,	off,	in_A
	out_A,	in_B,	in_A
	in_B,	off,	in_A
	in_B,	out_A,	in_A
	out_B,	off,	in_A
	off,	off,	out_A
	off,	in_A,	out_A
	off,	in_B,	out_A
	off,	out_B,	out_A
	in_A,	off,	out_A
	in_A,	in_B,	out_A
	in_A,	out_B,	out_A
	in_B,	off,	out_A
	in_B,	in_A,	out_A
	out_B,	off,	out_A
	out_B,	in_A,	out_A
	off,	off,	in_B
	off,	in_A,	in_B
	off,	out_A,	in_B
	off,	out_B,	in_B
	in_A,	off,	in_B
	in_A,	out_B,	in_B
	out_A,	off,	in_B
	out_B,	off,	in_B
	out_B,	in_A,	in_B
	off,	off,	out_B
	off,	in_A,	out_B
	off,	out_A,	out_B
	off,	in_B,	out_B
	in_A,	off,	out_B
	in_A,	in_B,	out_B
	out_A,	off,	out_B
	out_A,	in_B,	out_B
	in_B,	off,	out_B
	in_B,	in_A,	out_B
	in_B,	out_A,	out_B

Tested with oscilloscope, 1PPS generator and ts2phc.
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Signed-off-by: Piotr Kwapulinski <piotr.kwapulinski@intel.com>
Tested-by: Ashish K <ashishx.k@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>