net: phy: micrel: Add support for PTP_PF_EXTTS for lan8841

Extend the PTP programmable gpios to implement also PTP_PF_EXTTS
function. The pins can be configured to capture both of rising
and falling edge. Once the event is seen, then an interrupt is
generated and the LTC is saved in the registers.

This was tested using:
ts2phc -m -l 7 -s generic -f ts2phc.cfg

Where the configuration was the following:
[global]
ts2phc.pin_index  6

[eth2]

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>