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>
diff --git a/board/amlogic/p212/MAINTAINERS b/board/amlogic/p212/MAINTAINERS
index 07ca6f2..74ad371 100644
--- a/board/amlogic/p212/MAINTAINERS
+++ b/board/amlogic/p212/MAINTAINERS
@@ -4,5 +4,6 @@
 F:	board/amlogic/p212/
 F:	include/configs/p212.h
 F:	configs/khadas-vim_defconfig
+F:	configs/libretech-ac_defconfig
 F:	configs/libretech-cc_defconfig
 F:	configs/p212_defconfig