dt-bindings: net: cdns,macb: Add external REFCLK property

dt-bindings: net: cdns,macb: Add external REFCLK property

REFCLK can be provided by an external source so this should be exposed
by a DT property. The REFCLK is used for RMII and in some SoCs that use
this driver the RGMII 125MHz clk can also be provided by an external
source.

Signed-off-by: Ryan Wanner <Ryan.Wanner@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/d558467c4d5b27fb3135ffdead800b14cd9c6c0a.1752510727.git.Ryan.Wanner@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>