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/drivers/cpu/imx8_cpu.c b/drivers/cpu/imx8_cpu.c
index 6345cd0..502c8eb 100644
--- a/drivers/cpu/imx8_cpu.c
+++ b/drivers/cpu/imx8_cpu.c
@@ -100,7 +100,7 @@
 }
 #endif
 
-int cpu_imx_get_desc(struct udevice *dev, char *buf, int size)
+int cpu_imx_get_desc(const struct udevice *dev, char *buf, int size)
 {
 	struct cpu_imx_platdata *plat = dev_get_platdata(dev);
 	int ret, temp;
@@ -126,7 +126,7 @@
 	return 0;
 }
 
-static int cpu_imx_get_info(struct udevice *dev, struct cpu_info *info)
+static int cpu_imx_get_info(const struct udevice *dev, struct cpu_info *info)
 {
 	struct cpu_imx_platdata *plat = dev_get_platdata(dev);
 
@@ -135,7 +135,7 @@
 	return 0;
 }
 
-static int cpu_imx_get_count(struct udevice *dev)
+static int cpu_imx_get_count(const struct udevice *dev)
 {
 	ofnode node;
 	int num = 0;
@@ -157,7 +157,7 @@
 	return num;
 }
 
-static int cpu_imx_get_vendor(struct udevice *dev,  char *buf, int size)
+static int cpu_imx_get_vendor(const struct udevice *dev,  char *buf, int size)
 {
 	snprintf(buf, size, "NXP");
 	return 0;