arm64: zynqmp: Add new jtag distro boot command

This patch adds new jtag distro boot command to look for bootscript file in
DDR and execute it first incase of jtag bootmode.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>