dt-bindings: net: dsa: vsc73xx: add {rx,tx}-internal-delay-ps

Add a schema validator to vitesse,vsc73xx.yaml for MAC-level RGMII delays
in the CPU port. Additionally, valid values for VSC73XX were defined,
and a common definition for the RX and TX valid range was created.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>