k64f: change flash device name

Recent Zephyr changes the flash device name.  Change to match it.
diff --git a/boot/zephyr/targets/frdm_k64f.h b/boot/zephyr/targets/frdm_k64f.h
index 01926d5..590b1cb 100644
--- a/boot/zephyr/targets/frdm_k64f.h
+++ b/boot/zephyr/targets/frdm_k64f.h
@@ -19,7 +19,7 @@
  * @brief Bootloader device specific configuration.
  */
 
-#define FLASH_DRIVER_NAME		"KSDK_FLASH"
+#define FLASH_DRIVER_NAME		"MCUX_FLASH"
 #define FLASH_ALIGN			8
 #define FLASH_AREA_IMAGE_0_OFFSET	0x20000
 #define FLASH_AREA_IMAGE_0_SIZE		0x20000