suniv: move SKIP_LOWLEVEL_INIT_ONLY into Kconfig

So far we stated the lack of a lowlevel() init function for the
Allwinner F1C100s board by defining the respective SKIP_* symbol in the
board's defconfig. However we don't expect any *board* to employ such
low level code, so expect this to be never used for the ARMv5 Allwinner
SoCs.

Select the appropriate symbols in the Kconfig, so that we can remove
them from the defconfig, and avoid putting them in future defconfigs for
other boards.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>