watchdog: stm32: add dynamic prescaler support

This patch allows to define the max prescaler by compatible.
To set a large range of timeout, the prescaler should be set
dynamically (from the timeout request) to improve the resolution
in order to have a timeout close to the expected value.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>