ArmVirtPkg/HighMemDxe: move to FDT client protocol

ArmVirtPkg/HighMemDxe: move to FDT client protocol

Use the FDT client protocol rather than parsing the DT directly using
fdtlib. While we're at it, update the code so it deals correctly with
memory nodes that describe multiple disjoint regions in their "reg"
properties, and make the code work with #address-cells/#size-cells
properties of <1> as well as <2>.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>