dt-bindings: net: dwmac-sun8i: Clean up clock delay chain descriptions

The clock delay chains found in the glue layer for dwmac-sun8i are only
used with RGMII PHYs. They are not intended for non-RGMII PHYs, such as
MII external PHYs or the internal PHY. Also, a recent SoC has a smaller
range of possible values for the delay chain.

This patch reformats the delay chain section of the device tree binding
to make it clear that the delay chains only apply to RGMII PHYs, and
make it easier to add the R40-specific bits later.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>