board: add support for nexbox a95x

board: add support for nexbox a95x

Nexbox A95X is a low cost Android STB based around an Amlogic s905(x) SoC
(quad A53), 8/16GB eMMC and 1/2GB RAM.

Both the s905 (gxbb) and s905x (gxl) variant is supported.

This defconfig uses mainline Linux.  As the mainline support for s905 is
quite young, use the latest release candidate.  There is currently no
support for this board in U-Boot or sources available for the vendor U-Boot,
so the defconfig doesn't build a u-boot and instead uses the existing U-Boot
available in the eMMC.  This also means that we need to wrap the kernel as
uImage, which isn't suppported for aarch64.  Instead this is done in the
post-build script.

Mainline aarch64 defconfig builds most drivers modular, so add mdev for
module autoloading.

[Peter: explicitly set BR2_cortex_a53=y]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>