dt-bindings: net: Add binding for the external clock for TI WiLink

The external clock is provided to the TI WiLink combo chip and it's needed
for any of the transport interfaces. However let's make it optional to
avoid breaking existing platforms that yet doesn't specify the clock.

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>