dt-bindings: serial: 8250: allow clock 'uartclk' and 'reg' for nxp,lpc1850-uart

Allow clock 'uartclk' and 'reg' for nxp,lpc1850-uart to align existed
driver and dts. It is really old platform. Keep the same restriction for
others.

Allow dmas and dma-names property, which allow maxItems 4 because very old
platform (arch/arm/boot/dts/nxp/lpc/lpc18xx.dtsi) use duplicate "tx", "rx",
"tx", "rx" as dma-names.

Fix below CHECK_DTB warnings:
  arch/arm/boot/dts/nxp/lpc/lpc4337-ciaa.dtb: serial@40081000 (nxp,lpc1850-uart): clock-names: ['uartclk', 'reg'] is too long

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Acked-by: "Rob Herring (Arm)" <robh@kernel.org>
Link: https://lore.kernel.org/r/20250602142745.942568-1-Frank.Li@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/8250.yaml

@@ -49,6 +49,24 @@ allOf:
         - required: [ clock-frequency ]
         - required: [ clocks ]
 
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: nxp,lpc1850-uart
+    then:
+      properties:
+        clock-names:
+          items:
+            - const: uartclk
+            - const: reg
+    else:
+      properties:
+        clock-names:
+          items:
+            - const: core
+            - const: bus
+
 properties:
   compatible:
     oneOf:
@@ -142,9 +160,22 @@ properties:
 
   clock-names:
     minItems: 1
-    items:
-      - const: core
-      - const: bus
+    maxItems: 2
+    oneOf:
+      - items:
+          - const: core
+          - const: bus
+      - items:
+          - const: uartclk
+          - const: reg
+
+  dmas:
+    minItems: 1
+    maxItems: 4
+
+  dma-names:
+    minItems: 1
+    maxItems: 4
 
   resets:
     maxItems: 1
@@ -237,7 +268,9 @@ if:
   properties:
     compatible:
       contains:
-        const: spacemit,k1-uart
+        enum:
+          - spacemit,k1-uart
+          - nxp,lpc1850-uart
 then:
   required: [clock-names]
   properties: