dfu: add backend for MTD device

Add DFU backend for MTD device: allow to read
and write on all MTD device (NAND, SPI-NOR,
SPI-NAND,...)

For example :
> set dfu_alt_info "nand_raw raw 0x0 0x100000"
> dfu 0 mtd nand0

This MTD backend provides the same level than dfu nand
backend for NAND and dfu sf backend for SPI-NOR;
So it can replace booth of them but it also
add support of spi-nand.

> set dfu_alt_info "nand_raw raw 0x0 0x100000"
> dfu 0 mtd spi-nand0

The backend code is based on the "mtd" command
introduced by commit 5db66b3aee ("cmd: mtd:
add 'mtd' command")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

doc/README.dfu

@@ -21,6 +21,7 @@ Overview:
   - NAND
   - RAM
   - SF (serial flash)
+  - MTD (all MTD device: NAND, SPI-NOR, SPI-NAND,...)
 
   These DFU backends are also used by
   - the dfutftp (see README.dfutftp)
@@ -30,6 +31,7 @@ Configuration Options:
   CONFIG_DFU
   CONFIG_DFU_OVER_USB
   CONFIG_DFU_MMC
+  CONFIG_DFU_MTD
   CONFIG_DFU_NAND
   CONFIG_DFU_RAM
   CONFIG_DFU_SF
@@ -104,6 +106,13 @@ Commands:
 
       with <partid> is the MTD partition index
 
+  "mtd" (all MTD device: NAND, SPI-NOR, SPI-NAND,...)
+    cmd: dfu 0 mtd <dev>
+      with <dev> the mtd identifier as defined in mtd command
+      (nand0, nor0, spi-nand0,...)
+    each element in "dfu_alt_info" =
+      <name> raw <offset> <size>  raw access to mtd device
+
   <interface> and <dev> are absent:
     the dfu command to use multiple devices
     cmd: dfu 0 list
@@ -114,6 +123,7 @@ Commands:
        nand <dev>=<alt1>;....;<altN>
        ram <dev>=<alt1>;....;<altN>
        sf <dev>=<alt1>;....;<altN>
+       mtd <dev>=<alt1>;....;<altN>
 
 
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