ARM: dts: bcm2835-rpi-zero-w: Add bcm43438 serial slave

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

In order to keep a debug UART we need to switch to uart1.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>