net: macb: ptp: Switch to gettimex64() interface

net: macb: ptp: Switch to gettimex64() interface

The macb PTP support currently implements the `gettime64` callback to allow
to retrieve the hardware clock time. Update the implementation to provide
the `gettimex64` callback instead.

The difference between the two is that with `gettime64` a snapshot of the
system clock is taken before and after invoking the callback. Whereas
`gettimex64` expects the callback itself to take the snapshots.

To get the time from the macb Ethernet core multiple register accesses have
to be done. Only one of which will happen at the time reported by the
function. This leads to a non-symmetric delay and adds a slight offset
between the hardware and system clock time when using the `gettime64`
method. This offset can be a few 100 nanoseconds. Switching to the
`gettimex64` method allows for a more precise correlation of the hardware
and system clocks and results in a lower offset between the two.

On a Xilinx ZynqMP system `phc2sys` reports a delay of 1120 ns before and
300 ns after the patch. With the latter being mostly symmetric.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>