arm64: dts: rockchip: fix the memory size of PX5 Evaluation board

Commit 122682b2abb6 ("arm64: dts: rockchip: Add PX5 Evaluation board")
sets the memory size to 2 GB, but this board only has 1 GB DRAM, so change
it to the correct value here.

Fixes: 122682b2abb6 ("arm64: dts: rockchip: Add PX5 Evaluation board")
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>