arm64: dts: imx8mn-beacon: Set SAI5 MCLK direction to output for HDMI audio

The HDMI bridge chip fails to generate an audio source due to the SAI5
master clock (MCLK) direction not being set to output. This prevents proper
clocking of the HDMI audio interface.

Add the `fsl,sai-mclk-direction-output` property to the SAI5 node to ensure
the MCLK is driven by the SoC, resolving the HDMI sound issue.

Fixes: 1d6880ce ("arm64: dts: imx8mn-beacon: Add HDMI video with sound")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>