ArmVirtPkg/PrePi: move DRAM discovery code into PrePi

ArmVirtPkg/PrePi: move DRAM discovery code into PrePi

ArmVirtQemuKernel and ArmVirtXen use essentially the same code to
retrieve DRAM information from the DT /memory node at early boot,
and invoke it via the ArmPlatformPeiBootAction () hook exposed by
ArmPlatformLib. Let's move this code into the PrePi implementation
these platforms share between them (and not with any other platforms)
so we can eliminate another dependency on the messy ArmPlatformLib.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>