mfd: sgi-ioc3: Turn Kconfig option into a bool

Module builds of ioc3 fail with following errors:

ERROR: "spurious_interrupt" [drivers/mfd/ioc3.ko] undefined!
ERROR: "pci_find_host_bridge" [drivers/mfd/ioc3.ko] undefined!

Exporting pci_find_host_bridge got rejected by maintainer, so easiest
fix is to disable module building, which even makes sense since both
SGI Origin and Octane always contain at least one IOC3 chip.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>