net: dsa: microchip: add the support for set_ageing_time

KSZ9477 has the 11 bit ageing count value which is split across the two
registers. And LAN937x has the 20 bit ageing count which is also split
into two registers. Each count in the registers represents 1 second.
This patch add the support for ageing time for KSZ9477 and LAN937x
series of switch.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>