dt-bindings: clock: Add AST2500 RMII RCLK definitions

The AST2500 has an explicit gate for the RMII RCLK for each of the two
MACs.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>