Documentation: net: dsa: add details about SJA1110

Denote that the new switch generation is supported, detail its pin
strapping options (with differences compared to SJA1105) and explain how
MDIO access to the internal 100base-T1 and 100base-TX PHYs is performed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>