drm/radeon/kms: add support for internal thermal sensors (v3)

rv6xx/rv7xx/evergreen families supported; older asics did
not have an internal thermal sensor.

Note, not all oems use the internal thermal sensor, so it's
only exposed in cases where it is used.

Note also, that most laptops use an oem specific ACPI solution for
GPU thermal information rather than using the internal thermal
sensor directly.

v2: export millidegrees celsius, use hwmon device properly.
v3: fix Kconfig

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 5b7a1a4..4cab0c6 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -61,6 +61,7 @@
         select DRM_KMS_HELPER
         select DRM_TTM
 	select POWER_SUPPLY
+	select HWMON
 	help
 	  Choose this option if you have an ATI Radeon graphics card.  There
 	  are both PCI and AGP versions.  You don't need to choose this to