watchdog: s3c2410_wdt: Add max and min timeout values

The watchdog maximum timeout value is determined by the number of bits
for the interval timer counter, its source clock frequency, the number
of bits of the prescaler and maximum divider value.

This can be calculated with the following equation:

max_timeout = counter / (freq / (max_prescale + 1) / max_divider)

Setting a maximum timeout value will allow the watchdog core to refuse
user-space calls to the WDIOC_SETTIMEOUT ioctl that sets not supported
timeout values.

For example, systemd tries to set a timeout of 10 minutes on reboot to
ensure that the machine will be rebooted even if a reboot failed. This
leads to the following error message on an Exynos5422 Odroid XU4 board:

[  147.986045] s3c2410-wdt 101d0000.watchdog: timeout 600 too big

Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>