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/qemu/cpu.c b/arch/x86/cpu/qemu/cpu.c
index f40fb4d..9ce86b3 100644
--- a/arch/x86/cpu/qemu/cpu.c
+++ b/arch/x86/cpu/qemu/cpu.c
@@ -10,7 +10,7 @@
 #include <qfw.h>
 #include <asm/cpu.h>
 
-int cpu_qemu_get_desc(struct udevice *dev, char *buf, int size)
+int cpu_qemu_get_desc(const struct udevice *dev, char *buf, int size)
 {
 	if (size < CPU_MAX_NAME_LEN)
 		return -ENOSPC;
@@ -20,7 +20,7 @@
 	return 0;
 }
 
-static int cpu_qemu_get_count(struct udevice *dev)
+static int cpu_qemu_get_count(const struct udevice *dev)
 {
 	return qemu_fwcfg_online_cpus();
 }