dt-bindings: ethernet-phy: add optional brr-mode flag

There is a group of PHY chips supporting BroadR-Reach link modes in
a manner allowing for more or less identical register usage as standard
Clause 22 PHY.
These chips support standard Ethernet link modes as well, however, the
circuitry is mutually exclusive and cannot be auto-detected.
The link modes in question are 100Base-T1 as defined in IEEE802.3bw,
based on Broadcom's 1BR-100 link mode, and newly defined 10Base-T1BRR
(1BR-10 in Broadcom documents).

Add optional brr-mode flag to switch the PHY to BroadR-Reach mode.
Signed-off-by: Kamil Horák (2N) <kamilh@axis.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20240712150709.3134474-4-kamilh@axis.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/ethernet-phy.yaml

@@ -93,6 +93,14 @@ properties:
       the turn around line low at end of the control phase of the
       MDIO transaction.
 
+  brr-mode:
+    $ref: /schemas/types.yaml#/definitions/flag
+    description:
+      If set, indicates the network cable interface is an alternative one as
+      defined in the BroadR-Reach link mode specification under 1BR-100 and
+      1BR-10 names. The PHY must be configured to operate in BroadR-Reach mode
+      by software.
+
   clocks:
     maxItems: 1
     description: