wdt: nuvoton: Add support for Nuvoton

Add watchdog controller driver for NPCM7xx/npcm8xx

the wdt design of npcm750 and npcm845 is the same.
so the driver can work on npcm750 and npcm845.
about npcm845 wdt dtsi i will followed kernel dts name
to use nuvoton,npcm750-wdt.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>