video: atmel_hlcdfb: Fix misaligned cache operation warning

Fix the warning,
 ---8<---
CACHE: Misaligned operation at range [3fdffff0, 3fdffffc]
 ---<8---

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>