watchdog: Add StarFive Watchdog driver

Add to support StarFive watchdog driver. The driver is imported from
linux kernel's drivers/watchdog/starfive-wdt.c without jh7100 support
because there is no support of jh7100 SoC in u-boot yet.
Howver, this patch has been kept the variant coding style because JH7100
can be added later and have a consistency with the linux driver.

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>