sun4i: Remove Cubieboard_512 board

It uses the exact same memory timings as the regular cubieboard, it just
has lower density dram chips.

get_ram_size will detect that only 512 MB is available on these boards, so
they will work fine with the regular Cubieboard board config.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>