ArmVirtPkg: implement ArmVirtPsciResetSystemPeiLib

ArmVirtPkg: implement ArmVirtPsciResetSystemPeiLib

Implement a ArmVirtPkg specific version of the PSCI ResetSystemLib that
is usable in the PEI phase, as the existing one relies on the FDT client
protocol, making it unsuitable.

Note that accessing the device tree passed by QEMU via its initial base
address is guaranteed to be safe at any time during the PEI phase, so we
can defer discovery of the PSCI method until the time the reset library
is actually invoked (which is rarely)

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2560
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>