dt-bindings: serial: 8250: Make clocks and clock-frequency exclusive

The 8250 binding before converting to json-schema states,

- clock-frequency : the input clock frequency for the UART
or
- clocks phandle to refer to the clk used as per Documentation/devicetree

for clock-related properties, where "or" indicates these properties
shouldn't exist at the same time.

Additionally, the behavior of Linux's driver is strange when both clocks
and clock-frequency are specified: it ignores clocks and obtains the
frequency from clock-frequency, left the specified clocks unclaimed. It
may even be disabled, which is undesired most of the time.

But "anyOf" doesn't prevent these two properties from coexisting, as it
considers the object valid as long as there's at LEAST one match.

Let's switch to "oneOf" and disallows the other property if one exists,
precisely matching the original binding and avoiding future confusion on
the driver's behavior.

Fixes: e69f5dc623f9 ("dt-bindings: serial: Convert 8250 to json-schema")
Cc: stable <[e-mail]>
Signed-off-by: Yao Zi <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/8250.yaml

···
                   - ns16550
                   - ns16550a
     then:
-      anyOf:
+      oneOf:
         - required: [ clock-frequency ]
         - required: [ clocks ]