ArmVirtPkg: ArmVirtPlatformLib: find the lowest memory node

While QEMU NUMA support on ARM will introduce more than one /memory node
in the device tree, it needs to find the lowest one and set
PcdSystemMemorySize with the actual size of this memory node.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19123 6f19259b-4bc3-4df7-8a09-765794883524