dt-bindings: net: nfc: s3fwrn5: Add optional clock

On some systems, S3FWRN5 depends on having an external clock enabled
to function correctly. Allow declaring that clock in the device tree.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>