tools: mkimage: Add support for initialization table for Zynq and ZynqMP

The Zynq/ZynqMP boot.bin file contains a region for register initialization
data. Filling in proper values in this table can reduce boot time
(e.g. about 50ms faster on QSPI boot) and also reduce the size of
the SPL binary.

The table is a simple text file with register+data on each line. Other
lines are simply skipped. The file can be passed to mkimage using the
"-R" parameter.

It is recommended to add reg init file to board folder.
For example:
CONFIG_BOOT_INIT_FILE="board/xilinx/zynqmp/xilinx_zynqmp_zcu102/reg.int

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>