net: dsa: qca8k: add set_ageing_time support

qca8k support setting ageing time in step of 7s. Add support for it and
set the max value accepted of 7645m.
Documentation talks about support for 10000m but that values doesn't
make sense as the value doesn't match the max value in the reg.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>