phy: tegra: xusb: Protect Tegra186 soc with config

As xusb-tegra186.c will be reused for Tegra194, it would be good to
protect Tegra186 soc data with CONFIG_ARCH_TEGRA_186_SOC. This commit
also reshuffles Tegra186 soc data single CONFIG_ARCH_TEGRA_186_SOC
will be sufficient.

Signed-off-by: JC Kuo <jckuo@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>