dt-bindings: net: microchip,ksz: document microchip,rmii-clk-internal

Add documentation for selecting reference rmii clock on KSZ88X3 devices

Signed-off-by: Ante Knezic <ante.knezic@helmholz.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>