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: NSeung-Woo Kim <sw0312.kim@samsung.com>