net: dsa: motorcomm: Identify port type at runtime

The yt921x_port_is_internal/external() macros hardcoded the port layout:
they treat ports 8 and 9 as external on every chip and count the dummy
ports as internal.

Identify the actual port capabilities using chip info and track
configuration of external ports (although only SerDes is supported now).
This is a hardening and cleanup step ahead of the SerDes PCS support, no
behavior changes intended.

Signed-off-by: David Yang <mmyangfl@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260923021945.2203368-4-mmyangfl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>