Proposal: U-Boot MTD (rewriting old story)

[Jagan Teki]

Yes, this is the old discussion and triggered again now since we had a
discussion on the last U-Boot contributor call.

I will brief the main points here, as most of the details are
mentioned in previous threads.

Here are the couple of old and new implementations on the SPI-NOR side.

http://u-boot.10912.n7.nabble.com/PATCH-v10-00-27-dm-Generic-MTD-Subsystem-with-SPI-NOR-interface-tt315694.html#none
http://u-boot.10912.n7.nabble.com/PATCH-0-5-mtd-Implement-MTD-UCLASS-use-SPINOR-td419538.html#a419541
https://patchwork.ozlabs.org/project/uboot/cover/20200709111709.68904-1-jagan@amarulasolutions.com/

Idea is to
1. Create MTD_UCLASS, already present
2. Create DM MTD Operations, above patchset does.
3. Create Flash specific MTD UClass's like
    UCLASS_NAND
    UCLASS_SPI_NAND
    UCLASS_SPI_NOR
    UCLASS_NOR
    UCLASS_UBI
and register to UCLASS_MTD as interface slaves like how MMC, SCSI are
registered to UCLASS_BLK
4. Sync only respective flash specific code, and drop __UBOOT__
5. Interface mtd, sf, and nand commands to UCLASS_MTD.

Hard points:
1. Hard to arrange Linux specific code and drop unneeded _UBOOT_ code,
but possible.
2. Requires a lot of testing.

Any inputs?
Jagan.