usb: phy: Add Kconfig entry for Tegra PHY driver

Tegra's EHCI driver has a build dependency on Tegra's PHY driver and
currently Tegra's PHY driver is built only when Tegra's EHCI driver is
built. Add own Kconfig entry for the Tegra's PHY driver so that drivers
other than ehci-tegra (like ChipIdea UDC) could work with ehci-tegra
driver being disabled in kernels config by allowing user to manually
select the PHY driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>