net: dsa: vsc73xx: make RGMII delays configurable

This patch switches hardcoded RGMII transmit/receive delay to
a configurable value. Delay values are taken from the properties of
the CPU port: 'tx-internal-delay-ps' and 'rx-internal-delay-ps'.

The default value is configured to 2.0 ns to maintain backward
compatibility with existing code.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>