lib: sparse: Make CHUNK_TYPE_RAW buffer aligned

CHUNK_TYPE_RAW buffer is not aligned, and flash sparse images by
fastboot will report "Misaligned operation" if DCACHE is enabled.

Flashing Sparse Image
CACHE: Misaligned operation at range [84000028, 84001028]
CACHE: Misaligned operation at range [84001034, 84002034]
CACHE: Misaligned operation at range [8401104c, 8401304c]

Fix it

Signed-off-by: qianfan Zhao <qianfanguijin@163.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>