opamp: introduce new mode

Some MCUs are supporting external resistor dividers for gain setting in
standalone mode.
Add OPAMP_FUNCTIONAL_MODE_STANDALONE to the opamp_functional_mode enum.

Signed-off-by: Alexander Kozhinov <[email protected]>

Author: KozhinovAlexander

dts/bindings/opamp/opamp-controller.yaml

@@ -13,6 +13,7 @@ properties:
       - "inverting"
       - "non_inverting"
       - "follower"
+      - "standalone"
     description: |
       Selects opamp functional mode.
 

include/zephyr/dt-bindings/opamp/opamp.h

@@ -19,6 +19,12 @@ enum opamp_functional_mode {
 	OPAMP_FUNCTIONAL_MODE_NON_INVERTING,
 	/** Follower mode */
 	OPAMP_FUNCTIONAL_MODE_FOLLOWER,
+	/**
+	 * @brief Standalone mode.
+	 * The gain is set by external resistors. The API call to set the gain
+	 * is ignored in this mode or has no impact.
+	 */
+	OPAMP_FUNCTIONAL_MODE_STANDALONE,
 };
 
 #endif /* ZEPHYR_INCLUDE_DT_BINDINGS_OPAMP_OPAMP_H_ */