net: dsa: mv88e6xxx: support 4000ps cycle counter period

The MV88E6393X family of devices can run its cycle counter off
an internal 250MHz clock instead of an external 125MHz one.

Add support for this cycle counter period by adding another set
of coefficients and lowering the periodic cycle counter read interval
to compensate for faster overflows at the increased frequency.

Otherwise, the PHC runs at 2x real time in userspace and cannot be
synchronized.

Fixes: de776d0d ("net: dsa: mv88e6xxx: add support for mv88e6393x family")
Signed-off-by: Shenghao Yang <me@shenghaoyang.info>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>