pinctrl: uniphier: support pin configuration for dedicated pins

PH1-LD4 and PH1-sLD8 SoCs have pins that support pin configuration
(pin biasing, drive strength control), but not pin-muxing.

Allow to fill the mux value table with -1 for those pins; pins with
mux value -1 will be skipped in the pin-mux set function.  The mux
value type should be changed from "unsigned" to "int" in order to
accommodate -1 as a special case.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>