net: dsa: mv88e6xxx: implement watchdog_ops for mv88e6250

The MV88E6352_G2_WDOG_CTL_* bits almost, but not quite, describe the
watchdog control register on the mv88e6250. Among those actually
referenced in the code, only QC_ENABLE differs (bit 6 rather than bit
5).

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>