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This is used by the AT25F series (only?), but is generic enough to
reside in spi25.c. The only currently supported chip is the AT25F512B.
Other members of that series need some additional infrastructure code,
hence this patch adds the erase function to the AT25F512B only.

Corresponding to flashrom svn r1600.
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>