phy: Add driver for mixel mipi dphy found on NXP's i.MX8 SoCs

This adds support for the Mixel DPHY as found on i.MX8 CPUs but since
this is an IP core it will likely be found on others in the future. So
instead of adding this to the nwl host driver make it a generic PHY
driver.

The driver supports the i.MX8MQ. Support for i.MX8QM and i.MX8QXP can be
added once the necessary system controller bits are in via
mixel_dphy_devdata.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Co-developed-by: Robert Chiras <robert.chiras@nxp.com>
Signed-off-by: Robert Chiras <robert.chiras@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>