amlogic: g12a: Add support for the S905X2 (G12A) platform ...

Introduce the preliminary support for the Amlogic S905X2 (G12A) SoC.

This port is a minimal implementation of BL31 capable of booting
mainline U-Boot and Linux. Tested on a SEI510 board.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Change-Id: Ife958f10e815a4530292c45446adb71239f3367f