Bluetooth: hci_ll: Add support for the external clock

Add support to manage the external clock provided to the WiLink combo chip
as it's needed for any of the transport interfaces.

To avoid breaking platforms not yet specifying the external clock, we make
it optional. In case the clock is successfully fetched during ->probe(),
let's manage it via the ->open|close() callbacks, to make sure the device
get properly powered on/off.

Fixes: ea452678

 ("arm64: dts: hikey: Fix WiFi support")
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>