rockchip: pinctrl: rk3399: add gmac io strength support

GMAC controller need to init the tx io driver strength to 13mA,
just like the description in dts pinctrl node, or else the controller
may only work in 100MHz Mode, and fail to work at 1000MHz mode.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich &lt;philipp.tomsich@theobroma-systems.com <mailto:philipp.tomsich@theobroma-systems.com&gt;>
Reviewed-by: Simon Glass <sjg@chromium.org>