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>