watchdog: booke_wdt: add __ro_after_init to booke_wdt_info

The object booke_wdt_info of watchdog_info structure is not
modified after getting initialized by booke_wdt_init. Apart from getting
referenced in init it is also stored in the info field of watchdog_device
structure which is of type const struct watchdog_info *info. So, it
becomes read only after init and therefore add __ro_after_init to it's
declaration.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>