net: dsa: ks8995: Add basic switch set-up

net: dsa: ks8995: Add basic switch set-up

We start to extend the KS8995 driver by simply registering it
as a DSA device and implementing a few switch callbacks for
STP set-up and such to begin with.

No special tags or other advanced stuff: we use DSA_TAG_NONE
and rely on the default set-up in the switch with the special
DSA tags turned off. This makes the switch wire up properly
to all its PHY's and simple bridge traffic works properly.

After this the bridge DT bindings are respected, ports and their
PHYs get connected to the switch and react appropriately through
the phylib when cables are plugged in etc.

Tested like this in a hacky OpenWrt image:

Bring up conduit interface manually:
ixp4xx_eth c8009000.ethernet eth0: eth0: link up,
  speed 100 Mb/s, full duplex

spi-ks8995 spi0.0: enable port 0
spi-ks8995 spi0.0: set KS8995_REG_PC2 for port 0 to 06
spi-ks8995 spi0.0 lan1: configuring for phy/mii link mode
spi-ks8995 spi0.0 lan1: Link is Up - 100Mbps/Full - flow control rx/tx

PING 169.254.1.1 (169.254.1.1): 56 data bytes
64 bytes from 169.254.1.1: seq=0 ttl=64 time=1.629 ms
64 bytes from 169.254.1.1: seq=1 ttl=64 time=0.951 ms

I also tested SSH from the device to the host and it works fine.

It also works fine to ping the device from the host and to SSH
into the device from the host.

This brings the ks8995 driver to a reasonable state where it can
be used from the current device tree bindings and the existing
device trees in the kernel.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250813-ks8995-to-dsa-v1-4-75c359ede3a5@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>