Allow the sector size of the disk device to be configured using hints or
FDT data. The sector size must be a multiple of the device's page size.
If not configured, use the historical default of the device page size.

Setting the disk sector size to 512 or 4096 allows a variety of standard
filesystems to be used on the device. Of course you wouldn't want to be
writing frequently to a SPI flash chip like it was a disk drive, but for
data that gets written once (or rarely) and read often, using a standard
filesystem is a nice convenient thing.