ARM: dts: imx7: Use audio_mclk_post_div instead audio_mclk_root_clk

The audio_mclk_root_clk was added as a gate with the CCGR121 (0x4790),
but according to the reference manual, there is no such gate. Moreover,
the consumer driver of the mentioned clock might gate it and leave
the ECSPI2 (the true owner of that gate) hanging. So lets use the
audio_mclk_post_div, which is the parent.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>