ARM: dts: rockchip: add gmac info for rk3288

add gmac info in rk3288.dtsi for GMAC driver

changes since v2:
1. add drive-strength in the pinctrl settings

Signed-off-by: Roger Chen <roger.chen@rock-chips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>