ARM: dts: bcm2837-rpi-3-b: Add bcm43438 serial slave

Add BCM43438 (bluetooth) as a slave device of uart0 (pl011/ttyAMA0).
This allows to automatically insert the bcm43438 to the bluetooth
subsystem instead of relying on userspace helpers (hciattach).

Overwrite chosen/stdout-path to use 8250 aux uart as console.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Loic Poulain <loic.poulain@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>