dm: net: macb: Implement link speed change callback

dm: net: macb: Implement link speed change callback

At present the link speed change callback is a nop. According to
macb device tree bindings, an optional "tx_clk" is used to clock
the ethernet controller's TX_CLK under different link speed.

In 10/100 MII mode, transmit logic must be clocked from a free
running clock generated by the external PHY. In gigabit GMII mode,
the controller, not the external PHY, must generate the 125 MHz
transmit clock towards the PHY.

Signed-off-by: Bin Meng <[email protected]>
Reviewed-by: Lukas Auer <[email protected]>
Tested-by: Lukas Auer <[email protected]>
Acked-by: Joe Hershberger <[email protected]>