dt-bindings: serial: 8250: allow "main" and "uart" as clock names

There are two compatible strings defined in "8250.yaml" that require
two clocks to be specified, along with their names:
  - "spacemit,k1-uart", used in "spacemit/k1.dtsi"
  - "nxp,lpc1850-uart", used in "lpc/lpc18xx.dtsi"

When only one clock is used, the name is not required.  However there
are two places that do specify a name:
  - In "mediatek/mt7623.dtsi", the clock for the "mediatek,mtk-btif"
    compatible serial device is named "main"
  - In "qca/ar9132.dtsi", the clock for the "ns8250" compatible
    serial device is named "uart"

In commit d2db0d7 ("dt-bindings: serial: 8250: allow clock
'uartclk' and 'reg' for nxp,lpc1850-uart"), Frank Li added the
restriction that two named clocks be used for the NXP platform
mentioned above.

Change that logic, so that an additional condition for (only) the
SpacemiT platform similarly restricts the two clocks to have the
names "core" and "bus".

Finally, add "main" and "uart" as allowed names when a single clock is
specified.

Fixes: 2c0594f ("dt-bindings: serial: 8250: support an optional second clock")
Cc: stable <stable@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507160314.wrC51lXX-lkp@intel.com/
Signed-off-by: Alex Elder <elder@riscstar.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250813031338.2328392-1-elder@riscstar.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: alexelder

Documentation/devicetree/bindings/serial/8250.yaml

@@ -59,7 +59,12 @@ allOf:
           items:
             - const: uartclk
             - const: reg
-    else:
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: spacemit,k1-uart
+    then:
       properties:
         clock-names:
           items:
@@ -183,6 +188,9 @@ properties:
     minItems: 1
     maxItems: 2
     oneOf:
+      - enum:
+          - main
+          - uart
       - items:
           - const: core
           - const: bus