dt-bindings: watchdog: sprd,sp9860-wdt: convert to YAML

Convert Spreadtrum SP9860 watchdog timer bindings to DT schema.
Adjust file name to match compatible.

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Acked-by: Chunyan Zhang <zhang.lyra@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/ZdDUlGdqH7Qv3SDu@standask-GA-A55M-S2HP


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>