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>
sun7i: Drop FAST_MBUS

We've several bug reports indicating that this causes stability issues,
so lets just drop it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
cubieboard2: Drop dram clock to 432 MHz

We've several reports indicating that 480 MHz causes stability issues.

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