dt-bindings: serial: sc16is7xx: Add description for polling mode

Polling mode is enabled if the "interrupts" property is missing.
Thus, this commit deletes "interrupts" entry from "required" section
and adds a description for the fallback to polling mode at the
"interrupts" entry.

Signed-off-by: Andre Werner <andre.werner@systec-electronic.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250110073104.1029633-1-andre.werner@systec-electronic.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>