boards: Amlogic: Add support for Libretech-AC

LibreTech AC is a single board computer manufactured by Libre Technology
with the following specifications:

 - Amlogic S805X ARM Cortex-A53 quad-core SoC @ 1.2GHz
 - ARM Mali 450 GPU
 - 512MiB DDR4 SDRAM
 - 10/100 Ethernet
 - HDMI 2.0 4K/60Hz display
 - 40-pin GPIO header
 - 4 x USB 2.0 Host
 - eMMC, SPI NOR Flash
 - Infrared receiver

The u-boot specific code is the same as the P212 support,
so use the P212 board support code with a distinct defconfig
and config include files.

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