adc: Add drivers infrastructure

Only Kconfig file for now.

Change-Id: I633ac209583b2fe6cbed3c75036ac245f0058df6
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 14dbc38..bb02b9c 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -61,4 +61,6 @@
 
 source "drivers/pinmux/Kconfig"
 
+source "drivers/adc/Kconfig"
+
 endmenu