MLK-25255-3: arm64: dts: imx8mm: add clock for rpmsg audio

Add necessary clock for rpmsg audio. These clocks are needed
by M4 image

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Peng Zhang <peng.zhang_8@nxp.com>
(cherry-picked from commit df9a1745901e59ecaa94a93d159d263c16a75549)