net: dsa: Add DSA driver for Hirschmann Hellcreek switches

Add a basic DSA driver for Hirschmann Hellcreek switches. Those switches are
implementing features needed for Time Sensitive Networking (TSN) such as support
for the Time Precision Protocol and various shapers like the Time Aware Shaper.

This driver includes basic support for networking:

 * VLAN handling
 * FDB handling
 * Port statistics
 * STP
 * Phylink

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>