watchdog: simatic: Use idiomatic selection of P2SB

While it's pretty much theoretical to be otherwise, make sure
that P2SB is selected only for X86. This is idiomatic dependency
which is used by all others who select it. Use it for Simatic
as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230822141859.2139630-1-andriy.shevchenko@linux.intel.com


Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>