schemas: i2c: add optional GPIO binding for SMBALERT# line

Most I2C controllers do not have a dedicated pin for SMBus Alerts. Allow
them to define a GPIO as a side-channel.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240909105835.28531-1-wsa+renesas@sang-engineering.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

dtschema/schemas/i2c/i2c-controller.yaml

@@ -135,6 +135,11 @@ properties:
       use this information to detect a stalled bus more reliably, for example.
       Can not be combined with 'multi-master'.
 
+  smbalert-gpios:
+    maxItems: 1
+    description:
+      Specifies the GPIO used for the SMBALERT# line. Optional.
+
   smbus:
     type: boolean
     description: