phy: add phy_get_bus_width()/phy_set_bus_width() calls

This adds a pair of APIs that allows the generic PHY subsystem to
provide information on the PHY bus width. The PHY provider driver may
use phy_set_bus_width() to set the bus width that the PHY supports.
The controller driver may then use phy_get_bus_width() to fetch the
PHY bus width in order to properly configure the controller.

Signed-off-by: Matt Porter <mporter@linaro.org>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>