usb: phy: move all PHY drivers to drivers/usb/phy/

that's a much more reasonable location for
those drivers. It helps us saving drivers/usb/otg/
for when we actually start adding generic OTG
code.

Also completely delete drivers/usb/otg/ as there's
nothing left there.

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