realtek: mdio: add reverse lookup bus/phy to port

realtek: mdio: add reverse lookup bus/phy to port

Until now there is a O(n) loop that looks up a port for a
given bus/phy combination. This is slow for high port count
devices (RTL839x and RTL931x). Implement a efficient reverse
lookup table for that.

While we are here adapt tiny bits of the documentation to
better reflect the driver logic after the recent refactoring.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/23230
Signed-off-by: Robert Marko <robimarko@gmail.com>