clk: rockchip: clk_rk3368: Implement "assign-clock-parent"

Implement the setting parent for gmac clock, and add internal
pll div set for mac clk.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>