zephyr: struct device const-ification

Follows up the change added to Zephyr with:

https://github.com/zephyrproject-rtos/zephyr/pull/24873

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index c77f96c..ffa4500 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -27,9 +27,9 @@
 #error "FLASH_DEVICE_ID could not be determined"
 #endif
 
-static struct device *flash_dev;
+static const struct device *flash_dev;
 
-struct device *flash_device_get_binding(char *dev_name)
+const struct device *flash_device_get_binding(char *dev_name)
 {
     if (!flash_dev) {
         flash_dev = device_get_binding(dev_name);