drivers: net: add Aspeed MDIO driver

Add a driver for the MDIO interface for Aspeed AST2600 SOC.  The driver
only supports clause 22 for now.

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