arm: dts: add watchdog device to NPCM750 device tree

Add watchdog device node to a common device tree for all Nuvoton
NPCM750 BMCs and a board specific device tree for the NPCM750 (Poleg)
evaluation board.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>