i2c: tegra: Use relaxed versions of readl/writel

There is nothing to synchronize in regards to memory accesses for PIO
transfers and for DMA transfers the DMA API takes care of the syncing.

Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>