memory: tegra: Increase handshake timeout on Tegra20

Turned out that it could take over a millisecond under some circumstances,
like running on a very low CPU/memory frequency. TRM says that handshake
happens when there is a "safe" moment, but not explains exactly what that
moment is. Apparently at least memory should be idling and thus the low
frequency should be a reasonable cause for a longer handshake delay.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>