net: mscc: ocelot: extend watermark encoding function

The ocelot_wm_encode function deals with setting thresholds for pause
frame start and stop. In Ocelot and Felix the register layout is the
same, but for Seville, it isn't. The easiest way to accommodate Seville
hardware configuration is to introduce a function pointer for setting
this up.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>