net: ftgmac100: Add Aspeed AST2600 support

Add support of the MAC controller of Aspeed AST2600 SOC.  The MAC
controller is the same with AST2500, except it has stand-alone MDIO
hardware block.

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