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 5db66b3a ("cmd: mtd:
add 'mtd' command")

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