realtek: dts: convert to upstream switch notation

realtek: dts: convert to upstream switch notation

There is currently a difference how upstream and downstream define
the switch in the dts. Downstream holds the switch as a member
node below a root switchcore parent. Upstream uses the switch as
the parent.

Upstream:

  ethernet-switch@1b000000 {
    mdio-controller@ca00 { };
    ethernet { };
    ethernet-ports { };
  }

Downstream:

  switchcore@1b000000 {
    ethernet-switch {
      ethernet-ports { };
    };
    mdio-controller@ca00 { };
    ethernet { };
  }

Align downstream to upstream and merge the ethernet-switch into
the parent node. For this to work adapt the port lookup in the MDIO
and PCS driver.

Remark! With this commit the boot process will give the spurious
error message "rtl838x_eth 1b000000.ethernet-switch:ethernet eth0:
Failed to create a device link to DSA switch 1b000000.ethernet-switch"
This comes from the fact that the switch is the parent of the ethernet
device. Thus a link back from ethernet device to the switch is no
longer possible. Testing shows that the error is just cosmetic.

Link: https://github.com/openwrt/openwrt/pull/23599
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>