net: dsa: qca8k: rework rgmii delay logic and scan for cpu port 6

Future proof commit. This switch have 2 CPU ports and one valid
configuration is first CPU port set to sgmii and second CPU port set to
rgmii-id. The current implementation detects delay only for CPU port
zero set to rgmii and doesn't count any delay set in a secondary CPU
port. Drop the current delay scan function and move it to the sgmii
parser function to generalize and implicitly add support for secondary
CPU port set to rgmii-id. Introduce new logic where delay is enabled
also with internal delay binding declared and rgmii set as PHY mode.

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