ARM: devicetree: Add .dtb files to arch/arm/boot/.gitignore

Compiled device tree blobs shouldn't be committed in the kernel
tree, so ideally git should ignore them.

This patch will enable ignoring of any .dtb files which appear in
arch/arm/boot/

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>