memory: tegra: Replace readl-writel with mc_readl-mc_writel

There is no need for a memory barriers on reading/writing of register
values as we only care about the read/write order, hence let's use the
common helpers.

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