usb: add APIs to access host registers from Tegra PHY

As Tegra PHY driver needs to access one of the host registers,
added few APIs.

Signed-off-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
[swarren: moved assignment of phy->is_ulpi_phy to previous patch.]
Signed-off-by: Stephen Warren <swarren@nvidia.com>