ARM: dts: sunxi: bananapi-m2-plus: Increase BT UART speed

Bluetooth module on BananaPi M2 Plus can also be used for streaming
audio. However, for that case higher UART speed is required.

Add a max-speed property.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210116105228.847073-1-jernej.skrabec@siol.net