net: ftgmac100: Ungate RCLK for RMII on ASPEED MACs

The 50MHz RCLK has to be enabled before the RMII interface will function.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>