ARM: dts: Sync Amlogic G12A with Linux 5.3-rc1

Sync the Amlogic Meson G12A DT and Bindings file with the Linux 5.3-rc1
from the commit 5f9e832c1370 ("Linus 5.3-rc1").

Also remove the meson-g12a-u-boot.dtsi and meson-g12a-u200-u-boot.dtsi,
now conflicting with the main DT content.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Mark Kettenis <kettenis@openbsd.org>