clk: imx6q: disable non functional divider

The /2 divider between pll4_audio_div and pll4_post_div is not working
on imx6q. The frequency is not halved as reported by the clock tree
and measured on clko.
In the technical reference manual the divider was removed in revision 4.
It's also not listed in the imx6qp manual.

imx6dl manual mentions a divider for PLL4 and the according register
description. So keep the divider here.

Signed-off-by: Jan Remmet <j.remmet@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>