dt-bindings: net: renesas,ethertsn: Document default for delays

The internal delay properties are not mandatory and should have a
documented default value. The device only supports either no delay or a
fixed delay and the device reset default is no delay, document the
default as no delay.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>