net: macb: add capability to not set the clock rate

SAMA7G5's ethernet IPs TX clock could be provided by its generic clock or
by the external clock provided by the PHY. The internal IP logic divides
properly this clock depending on the link speed. The patch adds a new
capability so that macb_set_tx_clock() to not be called for IPs having
this capability (the clock rate, in case of generic clock, is set at the
boot time via device tree and the driver only enables it).

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>