arm: zynq: Define distro boot commnads for qspi, nand and nor

This patch adds distro boot commands for qspi, nand and nor.
The distro boot commands now reads the script from flash offset
of 15.75MB and executes it.

Setup default location via script_offset_f to 15.75MB to match the most
xilinx reference boards for qspi, nand and set the same using
script_offset_nor to 0xE2EC0000 for nor.
256kB 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>