usb: Add USB repeater generic framework

With more SoCs moving towards eUSB2, such platforms will have to use
a USB 2.0 compliance repeater. This repeater HW-wise usually deals with
level shifting, but depending on the implementation it can do much more.
So add a ganeric USB framework that would allow either a generic PHY or
some USB host controller to control and get a repeater from a devicetree
phandle. This framework will further be used by platform specific
drivers to register the repeater and implement platform specific ops.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>