net: dsa: qca8k: make rgmii delay configurable

The legacy qsdk code used a different delay instead of the max value.
Qsdk use 1 ns for rx and 2 ns for tx. Make these values configurable
using the standard rx/tx-internal-delay-ps ethernet binding and apply
qsdk values by default. The connected gmac doesn't add any delay so no
additional delay is added to tx/rx.
On this switch the delay is actually in ns so value should be in the
1000 order. Any value converted from ps to ns by dividing it by 1000
as the switch max value for delay is 3ns.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>