net: mdio: aspeed: Introduce read write function for c22 and c45

Add following additional functions to move out the implementation from
aspeed_mdio_read() and aspeed_mdio_write().

c22:
 - aspeed_mdio_read_c22()
 - aspeed_mdio_write_c22()

c45:
 - aspeed_mdio_read_c45()
 - aspeed_mdio_write_c45()

Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>