dt-bindings: net: dp83822: change ti,rmii-mode description

Drop reference to the 25MHz clock as it has nothing to do with connecting
the PHY and the MAC.
Add info about the reference clock direction between the PHY and the MAC
as it depends on the selected rmii mode.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240305141309.127669-1-jeremie.dautheribes@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>