usb: tegra: Fix typo in tegra20-usb-phy documentation

The device tree binding documentation for tegra20-usb-phy used to claim
that the vbus-supply property is required for host phys and optional
for otg phys. This should be the other way around.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>