ARM: imx: Enable DFU SF on all i.MX6 DHSOM and include dfu_alt_info in env

ARM: imx: Enable DFU SF on all i.MX6 DHSOM and include dfu_alt_info in env

Enable DFU SF on all i.MX6 DHSOM so the SPI NOR can be exposed via
DFU, and include dfu_alt_info which exposes the full SPI NOR via DFU.
To install new bootloader into the SPI NOR using DFU, it is necessary
to pad the u-boot-with-spl.imx to 0x400 Bytes offset first and then
send it to the board as follows:

"
u-boot=> dfu 0 sf
"

"
host$ dd if=/dev/zero of=pad.bin bs=1024 count=1
host$ cat pad.bin u-boot-with-spl.imx > flash.bin
host$ dfu-util -w -a 0 -D flash.bin
"

Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>