arm64: zynqmp: Setup multiboot register to 0

arm64: zynqmp: Setup multiboot register to 0

On Kria when board starts from Image A or Image B partition multiboot
register is already setup to that location. When reset command is called
board is issuing soft reset which start SW at already used location (offset
of multiboot * 32k).
But board should continue to run from multiboot offset 0 (start of QSPI)
and call early bootloader every reboot that's why clear multiboot register
to 0 by default to go that route all the time.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/edaf714a778bdd7447533a77b3455e4fd623f9da.1717420131.git.michal.simek@amd.com