integratorcp: adding vmstate for save/restore

VMState added by this patch preserves correct
loading of the integratorcp device state.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-id: 20170131114310.6768.79416.stgit@PASHA-ISP
[PMM: removed unnecessary minimum_version_id_old lines]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>