dt-bindings: serial: Remove obsolete nxp,lpc1850-uart.txt

nxp,lpc1850-uart.txt binding is already covered by 8250.yaml, so remove
it.

Reviewed-by: Conor Dooley <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Rob Herring <[email protected]>

Author: robherring