phy: tegra: xusb: Tegra210 host mode VBUS control

To support XUSB host controller ELPG, this commit moves VBUS control
.phy_power_on()/.phy_power_off() to .phy_init()/.phy_exit().
When XUSB host controller enters ELPG, host driver invokes
.phy_power_off(), VBUS should remain ON so that USB devices will not
disconnect. VBUS can be turned OFF when host driver invokes
.phy_exit() which indicates disabling a USB port.

Signed-off-by: JC Kuo <jckuo@nvidia.com>
Acked-By: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>