spl: spl_mmc: add __maybe_unused to mmc_load_image_raw_sector()

If there are no CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_PARTITION,
CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR and CONFIG_SPL_OS_BOOT,
there is unused-function build warning. Add __maybe_unused macro
to remove the warning.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>