net: dsa: mt7530: support setting ageing time

MT7530 has a global address age control register, so use it to set
ageing time.

The applied timer is (AGE_CNT + 1) * (AGE_UNIT + 1) seconds

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>