usb: phy: tegra: Use relaxed versions of readl/writel

There is nothing to synchronize in regards to memory stores, thus all
readl/writel occurrences in the code could be replaced with a relaxed
versions, for consistency.
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20200106013416.9604-9-digetx@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>