MLK-21635: ARM: imx6sll-evk: fix clock reparent issue

MLK-21635: ARM: imx6sll-evk: fix clock reparent issue

There is error log:

[    0.000000] clk: failed to reparent pll4_audio_div to osc: -22
[    0.000000] clk: failed to reparent pll4_audio_div to osc: -22

The reason is that the assigned-clocks is used in imx6sll.dtsi
clks node, so the assigned-clocks in imx6sll-evk.dts will override
the previous one, then cause the reparent error issue.

Redefine the assigned-clocks in imx6sll-evk.dts.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Viorel Suman <viorel.suman@nxp.com>