phy: tegra: xusb: Add set_mode support for USB 2 phy on Tegra210

Add support for set_mode on USB 2 phy. This allow XUSB host/device mode
drivers to configure the hardware to corresponding modes.

Signed-off-by: Nagarjuna Kristam <nkristam@nvidia.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>