ArmVirtPkg: switch to generic ResetSystemRuntimeDxe

For obscure reasons, ARM platforms use a different implementation of
the ResetSystem() runtime service call than other platforms. So let's
switch all ArmVirtPkg platforms to the generic version instead.

Given that all platforms use an implementation of EfiResetSystemLib [as
consumed by the ResetRuntimeDxe in EmbeddedPkg that we are replacing]
which is unlikely to be depended upon by out of tree platforms, let's
simply modify this library into an implementation of ResetSystemLib
instead [which is what the generic driver in MdeModulePkg consumes]

This does mean we need to update all clients at the same time, which
is why all changes are part of the same patch.

As before, warm reset and platform specific reset are mapped onto
cold reset (which is the only thing PSCI implements, at least the
version we depend on). The new library function EnterS3WithImmediateWake()
is left unimplemented, as permitted by the ResetSystemLib library class.

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