gitattributes: dont treat non-text files as text

With recent update in u-boot gitattributes all files are treated as regular
text files. This creates problems with special files and repo always
shows uncommitted files like below.

Your branch is up-to-date with 'origin/master'.
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   tools/logos/compulab.bmp
	modified:   tools/logos/denx-comp.bmp
	modified:   tools/logos/toradex.bmp

To fix above problem special files bmp/ttf files are treated as binary
files in the gitattributes.

Signed-off-by: Vignesh Rajendran <[email protected]>
Signed-off-by: Veeraiyan Chidambaram <[email protected]>

.gitattributes

@@ -1,2 +1,5 @@
 # Declare files that always have LF line endings on checkout
 * text eol=lf
+# Denote all files that are truly binary and should not be modified
+*.bmp binary
+*.ttf binary