dt-bindings: net: ftgmac100: Describe clock properties

Critically, the AST2600 requires ungating the RMII RCLK if e.g. NCSI is
in use.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>