dt-bindings: rtc: qcom-pm8xxx: add nvmem-cell offset

On many Qualcomm platforms the PMIC RTC control and time registers are
read-only so that the RTC time can not be updated. Instead an offset
needs be stored in some machine-specific non-volatile memory, which a
driver can take into account.

Add an 'offset' nvmem cell which can be used to store a 32-bit offset
from the Unix epoch so that the RTC time can be updated on such
platforms.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: David Collins <quic_collinsd@quicinc.com>
Link: https://lore.kernel.org/r/20230202155448.6715-17-johan+linaro@kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/qcom-pm8xxx-rtc.yaml

@@ -40,6 +40,16 @@ properties:
     description:
       Indicates that the setting of RTC time is allowed by the host CPU.
 
+  nvmem-cells:
+    items:
+      - description:
+          four-byte nvmem cell holding a little-endian offset from the Unix
+          epoch representing the time when the RTC timer was last reset
+
+  nvmem-cell-names:
+    items:
+      - const: offset
+
   wakeup-source: true
 
 required:
@@ -69,6 +79,8 @@ examples:
           compatible = "qcom,pm8921-rtc";
           reg = <0x11d>;
           interrupts = <0x27 0>;
+          nvmem-cells = <&rtc_offset>;
+          nvmem-cell-names = "offset";
         };
       };
     };