memory: tegra: Include io.h instead of iopoll.h

The register polling code was gone, but the included header change was
missed. Fix it up for consistency.

Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>