samples: driver: watchdog: Add GD32 SoCs support

Add support for boards that implement GD32 SoC.

Overlay file for longan_nano_lite enables FWDGT.

Add WDT_FEED_INTERVAL to make adjustable
the execution cycle of wdt_feed().

And also, make waiting for the WDT_FEED_INTERVAL period
for the watchdog window opens at the start of the test.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>