pinctrl: single: set function mask as optional

Since Hisilicon's pin controller is divided into two parts. One is the
function mux, and the other is pin configuration. These two parts are
in the different memory regions. So make pinctrl-single,function-mask
as optional property. Then we can define pingroups without valid
function mux that is only used for pin configuration.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>