ARM: dts: ast2600: Add MDIO devices

There are 4 MDIO bus controllers in AST2600 SOC.  Each of them can
connect to one or more PHY chips and is flexible to work with the 4 MAC
devices in AST2600.  On AST2600 EVB, MDIO 0,1,2,3 connect to the PHY
chips used by MAC 0,1,2,3 respectively.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>