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>

arch/x86/Kconfig

@@ -2433,9 +2433,19 @@ config X86_DMA_REMAP
 	depends on STA2X11
 
 config IOSF_MBI
-	tristate
-	default m
+	tristate "Intel System On Chip IOSF Sideband support"
 	depends on PCI
+	---help---
+	  Enables sideband access to mailbox registers on SoC's. The sideband is
+	  available on the following platforms. This list is not meant to be
+	  exclusive.
+	   - BayTrail
+	   - Cherryview
+	   - Braswell
+	   - Quark
+
+	  You should say Y if you are running a kernel on one of these
+	  platforms.
 
 config PMC_ATOM
 	def_bool y