Add a schema validator to nxp,sja1105.yaml and to dsa.yaml for explicit
MAC-level RGMII delays. These properties must be per port and must be
present only for a phy-mode that represents RGMII.

We tell dsa.yaml that these port properties might be present, we also
define their valid values for SJA1105. We create a common definition for
the RX and TX valid range, since it's quite a mouthful.

We also modify the example to include the explicit RGMII delay properties.
On the fixed-link ports (in the example, port 4), having these explicit
delays is actually mandatory, since with the new behavior, the driver
shouts that it is interpreting what delays to apply based on phy-mode.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>