mtd: spi-nor: add Kconfig option to disable 4K sectors

Current situation with 4K sectors is quite messy. First of all, some
MTD "users" don't work with such small size. An example may be UBIFS
which requires 15 KiB erase blocks as a minimum. In theory spi-nor
should provide multiple erase regions and MTD "users" should use the
one they need. Unforunately that is not implemented.

In the result our flashes database in spi-nor is hackish. For some
flashes we pretend they don't support 4K sectors just because some
distribution uses UBIFS on it. This ofc leads to conflicts, like
Samsung using w25q128 with 4K sectors vs. OpenWrt requiring it to
pretend it's 64 KiB blocks only.

My idea (plan?) for fixing this situation:
1) Use real hw info (this requires a way for disabling 4K for now)
2) Provide detailed info about erase regions
3) Make UBIFS work with devices that support 4K sectors

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>