dt-bindings: net: dsa: hellcreek: add missing CPU port phy-mode/fixed-link to example

Looking at hellcreek_phylink_get_caps(), I see that depending on whether
is_100_mbits is set, speeds of 1G or of 100M will be advertised. The
de1soc_r1_pdata sets is_100_mbits to true.

The PHY modes declared in the capabilities are MII, RGMII and GMII. GMII
doesn't support 100Mbps, and as for RGMII, it would be a bit implausible
to me to support this PHY mode but limit it to only 25 MHz. So I've
settled on MII as a phy-mode in the example, and a fixed-link of
100Mbps.

As a side note, there exists such a thing as "rev-mii", because the MII
protocol is asymmetric, and "mii" is the designation for the MAC side
(expected to be connected to a PHY), and "rev-mii" is the designation
for the PHY side (expected to be connected to a MAC). I wonder whether
"mii" or "rev-mii" should actually be used here, since this is a CPU
port and presumably connected to another MAC.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>