spl_mmc: allow to load raw image

The function spl_parse_image_header() falls back to a raw image
if the U-Boot header is missing and CONFIG_SPL_PANIC_ON_RAW_IMAGE
is undefined.  While, mmc_load_image_raw_sector() only accepts a
U-Boot legacy image or an FIT image, preventing us from loading a
raw image.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>