dt-bindings: serial: 8250: move a constraint

dt-bindings: serial: 8250: move a constraint

A block that required a "spacemit,k1-uart" compatible node to
specify two clocks was placed in the wrong spot in the binding.
Conor Dooley pointed out it belongs earlier in the file, as part
of the initial "allOf".

Fixes: 2c0594f9f0629 ("dt-bindings: serial: 8250: support an optional second clock")
Cc: stable <stable@kernel.org>
Reported-by: Conor Dooley <conor@kernel.org>
Closes: https://lore.kernel.org/lkml/20250729-reshuffle-contented-e6def76b540b@spud/
Signed-off-by: Alex Elder <elder@riscstar.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250813032151.2330616-1-elder@riscstar.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>