net: ethernet: stmmac: demote warnings about missing optional clocks

The specification of a "eth-ck" and a "ptp_ref" clock is optional per
the binding and the driver handles them gracefully.
Demote the output to an info message accordingly.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>