cpu: Convert the methods to use a const udevice *

These functions should not modify the device. Convert them to const so
that callers don't need to cast if they have a const udevice *.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/ivybridge/model_206ax.c b/arch/x86/cpu/ivybridge/model_206ax.c
index 5954a24..55f7cc2 100644
--- a/arch/x86/cpu/ivybridge/model_206ax.c
+++ b/arch/x86/cpu/ivybridge/model_206ax.c
@@ -410,14 +410,15 @@
 	return 0;
 }
 
-static int model_206ax_get_info(struct udevice *dev, struct cpu_info *info)
+static int model_206ax_get_info(const struct udevice *dev,
+				struct cpu_info *info)
 {
 	return cpu_intel_get_info(info, INTEL_BCLK_MHZ);
 
 	return 0;
 }
 
-static int model_206ax_get_count(struct udevice *dev)
+static int model_206ax_get_count(const struct udevice *dev)
 {
 	return 4;
 }