watchdog: softdog: make pretimeout support a compile option

It occurred to me that the panic pretimeout governor will stall the
softdog, because it is purely software which simply breaks when the
kernel panics. Testing governors with the softdog on the other hand is
really useful, so make this feature a compile time option which nees to
be enabled explicitly. This also removes the overhead if pretimeout
support is not used because it will now be compiled away (saving ~10% on
ARM32).

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>