This adds support for the w25n01gv, a 128MB SPI JEDEC NAND flash device.
The JEDEC commands for SPI NAND are different enough for SPI NOR, so yes
it's worth a different driver.
I'm only implementing enough to do reads from devices with ECC already
enabled in hardware. There are plenty of other ways to configure
the chip both at manufacturing and runtime; I'm choosing to be simple
for now whilst we figure out how we should expose NAND storage devices
to upper layers.
Writes, erases, bad block remapping, etc - that can all come later
as we figure out how to expose those via GEOM.
Tested with AC1300 router, w25n01gv, checksummed contents against
what Linux also sees.