dt-bindings: rtc: add refclk to mpfs-rtc

The rtc on PolarFire SoC does not use the AHB clock as its reference
frequency, but rather a 1 MHz refclk that it shares with MTIMER. Add
this second clock to the binding as a required property.

Fixes: 4cbcc0d7b3 ("dt-bindings: rtc: add bindings for microchip mpfs rtc")
Reviewed-by: Daire McNamara <daire.mcnamara@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220413075835.3354193-7-conor.dooley@microchip.com
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/rtc/microchip,mfps-rtc.yaml

@@ -31,11 +31,19 @@ properties:
           to that of the RTC's count register.
 
   clocks:
-    maxItems: 1
+    items:
+      - description: |
+          AHB clock
+      - description: |
+          Reference clock: divided by the prescaler to create a time-based
+          strobe (typically 1 Hz) for the calendar counter. By default, the rtc
+          on the PolarFire SoC shares it's reference with MTIMER so this will
+          be a 1 MHz clock.
 
   clock-names:
     items:
       - const: rtc
+      - const: rtcref
 
 required:
   - compatible
@@ -48,11 +56,12 @@ additionalProperties: false
 
 examples:
   - |
+    #include "dt-bindings/clock/microchip,mpfs-clock.h"
     rtc@20124000 {
         compatible = "microchip,mpfs-rtc";
         reg = <0x20124000 0x1000>;
-        clocks = <&clkcfg 21>;
-        clock-names = "rtc";
+        clocks = <&clkcfg CLK_RTC>, <&clkcfg CLK_RTCREF>;
+        clock-names = "rtc", "rtcref";
         interrupts = <80>, <81>;
     };
 ...