ARM: dts: Limit BT modem baud rate on 3B

The 3B doesn't have the flow control signals connected to the BT modem,
which limits the maximum usable baud rate to below 1 Mbaud. Use
921600 as a relatively safe default value, but the krnbt_baudrate
parameter can still override it.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>