pinctrl: add ZTE ZX pinctrl driver support

The pin controller on ZTE ZX platforms is kinda of hybrid.  It consists
of a main controller and an auxiliary one.  For example, on ZX296718 SoC,
the main controller is TOP_PMM and the auxiliary one is AON_IOCFG.  Both
controllers work together to control pin multiplexing and configuration.

For most of pins, the pinmux function is controlled by main controller
only, and this type of pins are meant by term 'TOP pins'.  For other
pins, the pinmux is controlled by both main and auxiliary controllers,
as the available multiplexing functions for the pin spread in both
controllers.  This type of pins are called 'AON pins'.  Though pinmux
implementation is quite different, pinconf is same for both types of
pins.  Both are controlled by auxiliary controller, i.e. AON_IOCFG on
ZX296718.

The patch adds the ZTE ZX core pinctrl driver to support this hybrid
pin controller as well as ZX296718 SoC specific pin data.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>