arm64: zynqmp: Generate dfu_alt_info based on multiboot register

Commit 2c8a0921 ("arm64: zynqmp: Add multiboot support for SPL/SPI
offset calculation") enabled u-boot.itb offset calculation based on
multiboot register. It is also good to reflect location of u-boot.itb image
for dfu_alt generation.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/5abc5f3047ce586b4d2740b5eea775d063295e28.1702906896.git.michal.simek@amd.com