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 <vignesh.rajendran@in.bosch.com>
Signed-off-by: Veeraiyan Chidambaram <veeraiyan.chidambaram@in.bosch.com>