x86/iosf: Add Kconfig prompt for IOSF_MBI selection

x86/iosf: Add Kconfig prompt for IOSF_MBI selection

Fixes an error in having the iosf build as 'default m'. On X86 SoC's the iosf
sideband is the only way to access information for some registers, as opposed to
through MSR's on other Intel architectures. While selecting IOSF_MBI is
preferred, it does mean carrying extra code on non-SoC architectures. This
exports the selection to the user, allowing those driver writers to compile out
iosf code if it's not being built.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Link: http://lkml.kernel.org/r/1409175640-32426-2-git-send-email-david.e.box@linux.intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>