board: amlogic: remove p212 derivatives

The Khadas vim and the libretech aml-s905x-cc (aka Potato) derive
from amlogic s905x reference design (P212).

All the code in these board is a copy/paste from the p212, which is
tedious to maintain. This change use p212 u-boot board for all these
boards, while keeping a dedicated defconfig to customize the names
and device tree.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>