checkpatch: check for SPDX in linker scripts

Current script does not enforces the check for the SPDX tag in the
linker scripts.
Add the extension '.ld' in the OpenOCD specific part.

Change-Id: I1cb6bc52e9dd86d99a26393085c7e2c9e8bac11f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8475
Tested-by: jenkins

tools/scripts/checkpatch.pl

@@ -3730,6 +3730,8 @@ sub process {
 				# OpenOCD specific: Begin
 				} elsif ($realfile =~ /\.(am|cfg|tcl)$/) {
 					$comment = '#';
+				} elsif ($realfile =~ /\.(ld)$/) {
+					$comment = '/*';
 				# OpenOCD specific: End
 				}