Add documentation for optional use of external clock. All devices
supported by this driver can work with an external clock in sync mode.
Another mode, called Pulse Per Second (PPS) is supported only by adis1649x
devices. The mode is selected by using the "clock-names" property.

The pin which is used as external clock input is selected by using a
custom optional property called "adi,ext-clk-pin". If this field is left
empty, DIO2 is assigned as default external clock input pin.
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>