phy: bcm-ns-usb2: new driver for USB 2.0 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 an EHCI controller driver.
There are (just a few) registers being defined in bcma header. It's
because DMU/CRU registers will be also needed in other drivers. We will
need them e.g. in PCIe controller/PHY driver and at some point probably
in clock driver for BCM53573 chipset. By using include/linux/bcma/ we
avoid code duplication.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>