boards: amlogic: add Radxa Zero defconfig

Add a defconfig for the Radxa Zero SBC, using an Amlogic S905Y2 chip.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
[narmstrong: updated u200 MAINTAINERS & add missing CONFIG_SYS_LOAD_ADDR from defconfig]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>