memory: tegra: Use relaxed versions of readl/writel

There is no need for inserting of memory barriers to access registers of
Memory Controller. Hence use the relaxed versions of the accessors.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>