checkpatch: adapt shell script to the tool's new version

checkpatch: adapt shell script to the tool's new version

With the new checkpatch it's not possible to send in one shot a
set of patches through stdin because the Signed-off-by tag present
in each patch will trigger the error of duplicated Signed-off-by.

Use the new command-line flag '--git' to let checkpatch to extract
the patches from git and analyse them one-by-one.

While there, add the SPDX tag to the script.

Change-Id: I74791b627b8cd68f2d49146d15ae35bbc610e64e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6166
Tested-by: jenkins