dt-bindings: serial: 8250: correct Nuvoton NPCM850 compatible

dt-bindings: serial: 8250: correct Nuvoton NPCM850 compatible

Nuvoton NPCM850 UART is compatible with NPCM750, so add proper fallback
to match existing DTS and Linux driver.

Fixes: c8177f90b7c6 ("dt-bindings: serial: 8250: Add npcm845 compatible string")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230124091602.44027-3-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>