clk: ast2600: Revise MII interface delay

The clock delay of the RMII/RGMII interface is controlled by SCU340~35C.
These values are obtained by measurement and experiments so we simply
use macro to define them.

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