rockchip: rk3288: Revert MAC_TXCLK in pinctrl for GMAC

This reverts TXCLK toggling that was accidently dropped while reworking
commit 2454b719 ("rockchip: rk3288: Add pinctrl support for the gmac
ethernet interface"). So the TX clock is enabled and we can use
GMAC_ROCKCHIP in 1Gbps when basic PINCTRL support is enabled
(!PINTRL_FULL).

Fixes: 2454b719 ("rockchip: rk3288: Add pinctrl support for the...")
Signed-off-by: Romain Perier <romain.perier@collabora.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>