flash/nor/at91sam: implement flash bank deallocation for SAM series

flash/nor/at91sam: implement flash bank deallocation for SAM series

Microchip (former Atmel) SAM drivers allocate a struct per chip.

at91sam3, at91sam34:
Deallocate all chip structs from the list at once, on the first bank
deallocation.

at91samd and at91sam4l drivers do not handle more than one bank.
Convert them to simple driver_priv allocation and use
default_flash_free_driver_priv().

Change-Id: I49d7200f38a4568c7e12f306c27d1b1b72646736
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4416
Tested-by: jenkins