net: phy: realtek: Add optional external PHY clock

In some cases, the PHY can use an external clock source instead of a
crystal.

Add an optional clock in the phy node to make sure that the clock source
is enabled, if specified, before probing.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>