PM / devfreq: tegra: Replace readl-writel with relaxed versions

There is no need to insert memory barrier on each readl/writel
invocation, hence use the relaxed versions.

Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>