We have observed unstable link with a new BNT switch.

Add wide_ppm_offset parameter to allow the user to control the clock
ppm offset on the CX4 interface for better compatibility.  Default is
100ppm, setting it to 1 will increase it to 300ppm.  Change default
SerDes1 reference clock to external source.
Signed-off-by: Chien Tung <chien.tin.tung@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>