watchdog: softdog: Add options 'soft_reboot_cmd' and 'soft_active_on_boot'

Add module parameters 'soft_reboot_cmd' and 'soft_active_on_boot' for
customizing softdog configuration; config reboot command by assigning
soft_reboot_cmd, and set soft_active_on_boot to start up softdog
timer at module initialization stage.

Signed-off-by: Woody Lin <woodylin@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200708083218.3157213-1-woodylin@google.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>