arm64: dts: imx8mp-beacon-kit: Fix audio_pll2 clock

Commit 16c98452 ("arm64: dts: imx8mp: don't initialize audio clocks
from CCM node") removed the Audio clocks from the main clock node, because
the intent is to force people to setup the audio PLL clocks per board
instead of having a common set of rates since not all boards may use
the various audio PLL clocks for audio devices.

This resulted in an incorrect clock rate when attempting to playback
audio, since the AUDIO_PLL2 wasn't set any longer. Fix this by
setting the AUDIO_PLL2 rate inside the SAI3 node since it's the SAI3
that needs it.

Fixes: 16c98452

 ("arm64: dts: imx8mp: don't initialize audio clocks from CCM node")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>