arm64: dts: hikey: add WL1835 Bluetooth device node

This adds the serial slave device for the WL1835 Bluetooth interface.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>