armv8: remove no longer needed lowlevel_init.S

When we added Allwinner SoC support to ARMv8, we needed to pull in an
implementation of lowlevel_init() calling the C function s_init(), as
sunxi required it as this time.

The last few patches got rid of this bogus requirement, and as sunxi was
still the only user, we can now remove this lowlevel_init.S from ARMv8
altogether.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>