ArmVirtPkg/FdtClientDxe: add methods to iterate over memory nodes

ArmVirtPkg/FdtClientDxe: add methods to iterate over memory nodes

Add high level methods to iterate over all 'reg' properties of all DT
nodes whose device_type properties have the value "memory". Since we are
modifying the FdtClient protocol, update the protocol and the only existing
implementation at the same time.

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