net-next: dsa: add new driver for qca8xxx family

This patch contains initial support for the QCA8337 switch. It
will detect a QCA8337 switch, if present and declared in the DT.

Each port will be represented through a standalone net_device interface,
as for other DSA switches. CPU can communicate with any of the ports by
setting an IP@ on ethN interface. Most of the extra callbacks of the DSA
subsystem are already supported, such as bridge offloading, stp, fdb.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>