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>