clocksource/drivers/tegra: Restore base address before cleanup

We're adjusting the timer's base for each per-CPU timer to point to the
actual start of the timer since device-tree defines a compound registers
range that includes all of the timers. In this case the original base
need to be restore before calling iounmap to unmap the proper address.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>