cfi-flash: Added support to flash_real_protect for Atmel flash devices

Some of the flash memories produced by ATMEL start in read-only mode.
We need to unprotect it. This patch allows the AT49BV6416 to work with
cfi_flash memories. Tested in the at91rm9200ek board.

Signed-off-by: Rafael Campos Las Heras <rafael.campos@hanscan.com>
Signed-off-by: Stefan Roese <sr@denx.de>