arm64: dts: hikey: Add external clock to the Bluetooth node

To make the TI WiLink chip work again for Bluetooth, let's add the missing
external clock to the Bluetooth node, such the driver can deal properly
with it during power on/off.

Fixes: ea452678 ("arm64: dts: hikey: Fix WiFi support")
Cc: Wei Xu <xuwei5@hisilicon.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>