xilinx: zynqmp: Save multiboot as variable

Save multiboot register as u-boot variable. And use it as primary source
for composing dfu_alt_info for capsule update. If variable is not defined

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Link: https://lore.kernel.org/r/96556221443489c952717bcb340b4707901c9bdd.1635149450.git.michal.simek@xilinx.com