nyan-big: add padding to its file

Without this padding nyan-big ends at a blank screen on boot.
Details on how to get to this padding can be found in the README.chromium under Notes.

Signed-off-by: Tristan Bastian <tristan-c.bastian@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>