armv8: ls1043ardb_sdcard: prepare falcon boot

Due to a conflict with recent Primary Protected Application (PPA),
PPA cannot be loaded for SPL stage, falcon boot is not enabled by
default. With compatible PPA image, to enable falcon boot, activate
these Kconfig options in defconfig
CONFIG_SPL_FIT=y
CONFIG_SPL_FSL_LS_PPA=y
CONFIG_SPL_GZIP=y
CONFIG_SPL_LOAD_FIT=y
CONFIG_SPL_OS_BOOT=y
CONFIG_SPL_OF_LIBFDT=y

Because environment variables are not avaiable during SPL stage for
SD boot, set "boot_os=y" as default.

Signed-off-by: York Sun <york.sun@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>