drivers: flash: Add driver for AT45 compatible SPI flash chips

Add a driver that can handle several instances of AT45 family chips,
which are enabled by specifying DT nodes for them with the "compatible"
property set to "atmel,at45" and other required properties like JEDEC
ID, chip capacity, block and page size etc. configured accordingly.

The driver is only capable of using "power of 2" binary page sizes in
those chips and at initialization configures them to work in that mode
(unless it is already done).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>