x86: Add efi runtime reset

Our selftest will soon test the actual runtime reset function rather than
the boot time one. For this, we need to ensure that the runtime version
actually succeeds on x86 to keep our travis tests work.

So this patch implements an x86 runtime reset function. It is missing
shutdown functionality today, but OSs usually implement that via ACPI
and this function does more than the stub from before, so it's at least
an improvement.

Eventually we will want to have full DM functionality in runtime services.
But this fixes a travis failure and doesn't clutter the code too heavily, so
we should pull it in without the amazing new RTS DM framework.

Signed-off-by: Alexander Graf <agraf@suse.de>