arm64: zynqmp: Define distro boot commnads for qspi and nand

This patch adds distro boot commands for qspi and nand. The
distro boot commands now reads the script from flash offset
of 63.5MB and executes it.

Setup default location via script_offset_f to 63.5MB to match the most
xilinx reference boards. 512kB allocated space for script size
(script_size_f) should be more than enough to cover custom boot logic.

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