usb: phy: isp1301: give it a context structure

this patch is a small preparation to fix
isp1301 driver so that other platforms can
use it.

We're defining our private data structure
to represent this device and adding the
PHY to the PHY list.

Later patches will come implementing proper
PHY API and removing bogus code from ohci_nxp
and lpc32xx_udc drivers.

Signed-off-by: Felipe Balbi <balbi@ti.com>