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/baytrail/cpu.c b/arch/x86/cpu/baytrail/cpu.c
index 18e48ff..309a50a 100644
--- a/arch/x86/cpu/baytrail/cpu.c
+++ b/arch/x86/cpu/baytrail/cpu.c
@@ -150,7 +150,7 @@
 	return bclk * ((platform_info.lo >> 8) & 0xff);
 }
 
-static int baytrail_get_info(struct udevice *dev, struct cpu_info *info)
+static int baytrail_get_info(const struct udevice *dev, struct cpu_info *info)
 {
 	info->cpu_freq = tsc_freq();
 	info->features = 1 << CPU_FEAT_L1_CACHE | 1 << CPU_FEAT_MMU;
@@ -158,7 +158,7 @@
 	return 0;
 }
 
-static int baytrail_get_count(struct udevice *dev)
+static int baytrail_get_count(const struct udevice *dev)
 {
 	int ecx = 0;