block: m25p80: Extend address mode

Extend address mode allows to switch flash 16 MiB banks,
allowing user to access all flash sectors.
This access mode is used by u-boot.

Signed-off-by: Marcin Krzeminski <marcin.krzeminski@nokia.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1458719789-29868-5-git-send-email-marcin.krzeminski@nokia.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>