The generic ethernet-phy binding allows describing an external clock since
commit 350b7a25 ("dt-bindings: net: phy: Document support for external PHY clk")
for cases where the phy is not supplied by an oscillator but instead
by a clock from the host system.

And the old named "rmii-ref" clock from 2014 is only specified for phys
of the KSZ8021, KSZ8031, KSZ8081, KSZ8091 types.

So allow retrieving and enabling the optional generic clock on phys that
do not provide a rmii-ref clock.
Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231201150131.326766-3-heiko@sntech.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>