dm: core: ofnode: Add ofnode_read_bootscript_flash()

dm: core: ofnode: Add ofnode_read_bootscript_flash()

ofnode_read_bootscript_flash() reads bootscript address from
/options/u-boot DT node. bootscr-flash-offset and bootscr-flash-size
properties are read and values are filled. When bootscr-flash-size is not
defined, bootscr-flash-offset property is unusable that's why cleaned.
Both of these properties should be defined to function properly.

Also add test to cover this new function.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/08a3e6c09cce13287c69ad370e409e7f1766b406.1693465465.git.michal.simek@amd.com