clk: rockchip: rk3036: rename emac ext source clock

There is only support rmii in the RK3036, so we should use the correct
ext clock name as described in the TRM.

Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
[update dt-binding document as well]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>