ARM64: dts: meson-gx: fix ATF reserved memory region

Vendor firmware/uboot has different reserved regions depending on
firmware version, but current codebase reserves the same regions on
GXL and GXBB, so move the additional reserved memory region to common
.dtsi.

Found when putting a recent vendor u-boot on meson-gxbb-p200.

Suggested-by: Neil Armstrong <narmstrong@baylibre.com>
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Hilman <khilman@baylibre.com>