memory: tegra30-emc: Firm up suspend/resume sequence

The current code doesn't prevent race conditions of suspend/resume vs CCF.
Let's take exclusive control over the EMC clock during suspend in a way
that is free from race conditions.

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