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/apollolake/cpu.c b/arch/x86/cpu/apollolake/cpu.c
index aa7a3db..0a6d2ad 100644
--- a/arch/x86/cpu/apollolake/cpu.c
+++ b/arch/x86/cpu/apollolake/cpu.c
@@ -9,7 +9,7 @@
 #include <asm/cpu_common.h>
 #include <asm/cpu_x86.h>
 
-static int apl_get_info(struct udevice *dev, struct cpu_info *info)
+static int apl_get_info(const struct udevice *dev, struct cpu_info *info)
 {
 	return cpu_intel_get_info(info, INTEL_BCLK_MHZ);
 }