ACPI / PMIC: xpower: Add depends on IOSF_MBI to Kconfig entry

This is necessary to avoid compilation issues on non x86 systems (where the
asm/iosf_mbi.h header is not available) and on x86 systems in case IOSF_MBI
support is not enabled there.

Note that the AXP288 PMIC is connected through the LPSS i2c controller, so
either we have IOSF_MBI support selected through the X86_INTEL_LPSS option,
or we have a kernel where the OpRegion will never work anyways.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>