arm64: dts: imx8mm-beacon-som: Fix Choppy BT audio

When streaming bluetooth audio, the sound is choppy due to the
fact that the default baud rate of the HCI interface is too slow
to handle 16-bit stereo at 48KHz.

The Bluetooth chip is capable of up to 4M baud on the serial port,
so this patch sets the max-speed to 4000000 in order to properly
stream audio over the Bluetooth.

Fixes: 593816fa ("arm64: dts: imx: Add Beacon i.MX8m-Mini development kit")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>