For devices that use a programmable clock for the AVB reference clock,
the driver may need to enable them.  Add code to find the optional clock
and enable it when available.
Signed-off-by: NAdam Ford <aford173@gmail.com>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>