sf: stmicro: add geometrical info for N25Q256 from Micron

Note: because 4-byte addressing is not supported yet,
at the moment only the first 16MiB of the device are available.

Signed-off-by: Jérôme Carretero <cJ@zougloub.eu>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>