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

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>
(cherry-picked from git://git.kernel.org/pub/scm/linux/kernel/git/mmind/linux-rockchip.git
 commit 2f513bd9eac1a2642b40387d4f89c3987641e59b)

Change-Id: If6a64fa0eab3d76b90f122e63d30c1d7940b0a57
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>