watchdog: simatic-ipc-wdt: convert to use P2SB accessor

Since we have a common P2SB accessor in tree we may use it instead of
open coded variants.

Replace custom code by p2sb_bar() call.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>