bcm53xx: add BCM53573 Ethernet fix sent upstream for v6.6

It seems that DSA-based b53 driver never worked with BCM53573 SoCs and
BCM53125.

In case of swconfig-based b53 this fixes a regression. Switching bgmac
from using mdiobus_register() to of_mdiobus_register() resulted in MDIO
device (BCM53125) having of_node set (see of_mdiobus_register_phy()).
That made downstream b53 driver read invalid data from DT and broke
Ethernet support.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 79fd3e62b4)

target/linux/bcm53xx/patches-5.4/131-ARM-dts-BCM53573-Fix-Tenda-AC9-switch-CPU-port.patch

@@ -0,0 +1,28 @@
+From: =?UTF-8?q?Rafa=C5=82=20Mi=C5=82ecki?= <rafal@milecki.pl>
+Date: Sun, 23 Jul 2023 19:48:13 +0200
+Subject: [PATCH 1/3] ARM: dts: BCM53573: Fix Tenda AC9 switch CPU port
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Primary Ethernet interface is connected to the port 8 (not 5).
+
+Fixes: 64612828628c ("ARM: dts: BCM53573: Add Tenda AC9 switch ports")
+Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
+---
+ arch/arm/boot/dts/broadcom/bcm47189-tenda-ac9.dts | 4 ++--
+ 1 file changed, 2 insertions(+), 2 deletions(-)
+
+--- a/arch/arm/boot/dts/bcm47189-tenda-ac9.dts
++++ b/arch/arm/boot/dts/bcm47189-tenda-ac9.dts
+@@ -135,8 +135,8 @@
+ 			label = "lan4";
+ 		};
+ 
+-		port@5 {
+-			reg = <5>;
++		port@8 {
++			reg = <8>;
+ 			label = "cpu";
+ 			ethernet = <&gmac0>;
+ 		};