dt-bindings: serial: 8250: support an optional second clock

The SpacemiT UART driver requires a bus clock to be enabled in addition
to the primary function clock.  Add the option to specify two clocks
for an 8250-compatible UART, named "core" and "bus".  If both are needed,
require them to be named.

Signed-off-by: Alex Elder <elder@riscstar.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250409192213.1130181-2-elder@riscstar.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/8250.yaml

@@ -135,7 +135,16 @@ properties:
   clock-frequency: true
 
   clocks:
-    maxItems: 1
+    minItems: 1
+    items:
+      - description: The core function clock
+      - description: An optional bus clock
+
+  clock-names:
+    minItems: 1
+    items:
+      - const: core
+      - const: bus
 
   resets:
     maxItems: 1
@@ -224,6 +233,25 @@ required:
   - reg
   - interrupts
 
+if:
+  properties:
+    compatible:
+      contains:
+        const: spacemit,k1-uart
+then:
+  required: [clock-names]
+  properties:
+    clocks:
+      minItems: 2
+    clock-names:
+      minItems: 2
+else:
+  properties:
+    clocks:
+      maxItems: 1
+    clock-names:
+      maxItems: 1
+
 unevaluatedProperties: false
 
 examples: