soc/tegra: fuse: Unmap registers once they are not needed anymore

Both Chip ID and strapping registers are now read out during of APB MISC
initialization, the registers' mapping isn't needed anymore once registers
are read. Hence let's unmap registers once they are not needed anymore,
for consistency.

Suggested-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>