ipq40xx: add DSA switch driver

ipq40xx: add DSA switch driver

Qualcomm IPQ40xx SoC-s have a variant of QCA8337N switch built-in.

It shares most of the stuff with its external counterpart, however it is
modified for the SoC.
Namely, it doesn't have second CPU port (Port 6), so it has 6 ports
instead of 7.
It also has no built-in PHY-s but rather requires external PSGMII based
companion PHY-s (QCA8072 and QCA8075) for which it first needs to carry
out calibration before using them.
PSGMII has a SoC built-in PHY that is used to connect to the PHY-s which
unfortunately requires some magic values as the datasheet doesnt document
the bits that are being set or the register at all.

Since its built-in it is MMIO like other peripherals and doesn't have its
own MDIO bus but depends on the SoC provided one.

CPU connection is at Port 0 and it uses some kind of a internal connection
and no traditional RGMII/SGMII.
It also doesn't use in-band tagging like other qca8k switches so a shinfo
based tagger is used.

This is based on the current OpenWrt qca8k version that has been imported
from generic target.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>