tools: imximage: display DCD block offset, length

tools: imximage: display DCD block offset, length

These values can be used to sign a U-Boot image for use when
loading an image through the Serial Download Protocol (SDP).

Note that the address of 0x910000 is usable with the stock
configuration of imx_usb_loader on i.MX6 and i.MX7 SOCs:

https://github.com/boundarydevices/imx_usb_loader/blob/master/mx6_usb_work.conf#L3

Refer to the section on imx_usb_loader in this post for more
details:

https://boundarydevices.com/high-assurance-boot-hab-dummies/

Signed-off-by: Eric Nelson <eric@nelint.com>