ARM: imx6q: Add pll4_audio_div to clock tree

There's a pll4_audio_div clock, an extra divider for pll4, missing
in current clock tree, thus add it.

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>