flash: add w25n (nand flash) support

This adds read only support for the W25N series of flash parts.

Specifically starting with the W25N01GV, a 128MiB SPI NAND flash.

This doesn't currently support writing or erasing, as this requires
a NAND flash layer that we don't currently have. There are also
plenty of other commands that aren't currently supported - notably
maintaining the on-chip flash translation layer, flash wear statistics,
etc.

But read support is fine enough for now; it at least allows for
reading the boot / config / calibration flash on my ASUS IPQ4018 based
router.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D48979