watchdog: Add Nuvoton NPCM watchdog driver

watchdog: Add Nuvoton NPCM watchdog driver

The Nuvoton NPCM750 has a watchdog implemented as a single register
inside the timer peripheral.

This driver exposes that watchdog as a standard watchdog device with
coarse timeout intervals, limited by the combination of prescaler and
counter that is provided by the hardware. The calculation is taken from
the Nuvoton vendor tree.

The watchdog is left running if a bootloader had it going. The rate is
the one specified in the device tree, or the default value (obtained
from the datasheet).

There is a pre-timeout IRQ that is wired up. This timeout always occurs
1024 clocks before the timeout.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>