phy: Add a driver for dm816x USB PHY

Add a minimal driver for dm816x USB. This makes USB work on dm816x
without any other changes needed as it can use the existing musb_dsps
glue layer for the USB controller.

Note that this phy is different from dm814x and am335x.

Cc: Bin Liu <binmlist@gmail.com>
Cc: Brian Hutchinson <b.hutchman@gmail.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Matthijs van Duin <matthijsvanduin@gmail.com>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>