phy: add A3700 UTMI PHY driver

Marvell Armada 3700 SoC has two USB controllers, each of them being
wired to an internal UTMI PHY. Add a driver to control them.

Igal Liberman worked on supporting the PHY, I took the while 'register
configuration' from his work and rewrote almost entirely the
driver/bindings around it.

Co-developed-by: Igal Liberman <igall@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Igal Liberman <igall@marvell.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>