clk: rockchip: rename rk3228 sclk_macphy_50m to sclk_mac_extclk

The sclk_macphy_50m is confusing, the sclk_mac_extclk describes
a external clock  clearly.

Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>