NOR: add read() callback to struct flash_driver

Final target is to force bus_width size during CFI flash
read.
In this first step I need to replace default flash read
with flash specific implementation.
This patch introduces:
- flash_driver_read() layer;
- default_flash_read(), backward compatible;
- read() callback in struct flash_driver;
- proper initialization in every flash_driver instance.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>

src/flash/nor/avrf.c

@@ -497,6 +497,7 @@ struct flash_driver avr_flash = {
 	.erase = avrf_erase,
 	.protect = avrf_protect,
 	.write = avrf_write,
+	.read = default_flash_read,
 	.probe = avrf_probe,
 	.auto_probe = avrf_auto_probe,
 	.erase_check = default_flash_mem_blank_check,