doc: dt: mtd: stop referring to driver code for spi-nor IDs

Pull the supported chip names from drivers/mtd/devices/m25p80.c and stop
pointing readers to Linux code.

Also (although I see this habit repeated throughout the
Documentation/devicetree/bindings/ tree), stop using the title "driver"
in this file, when we're trying explicitly to describe hardware, not
software.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: <devicetree@vger.kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>