phy: bcm-ns-usb3: new driver for USB 3.0 PHY on Northstar

Northstar is a family of SoCs used in home routers. They have USB 2.0
and 3.0 controllers with PHYs that need to be properly initialized.
This driver provides PHY init support in a generic way and can be bound
with XHCI controller driver.

There aren't any public datasheets from Broadcom so we can't have nice
defines for all used bits. It means we just follow Broadcom's
initialization procedure using their magic values. We were quite lucky
actually that Broadcom put some comments in their SDK reference code
explaining what given writes are responsible for.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>