binman: add support for skipping file concatenation for mkimage

Some image types handled by mkimage require the datafiles to be passed
independently (-d data1:data2) for specific handling of each. A
concatenation of datafiles prior to passing them to mkimage wouldn't
work.

That is the case for rkspi for example which requires page alignment
and only writing 2KB every 4KB.

This adds the ability to tell binman to pass the datafiles without
prior concatenation to mkimage, by adding the multiple-data-files
boolean property to the mkimage node.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>