pinctrl: sirf: add DT-binding pinmux mapping support

This makes us possible to define pinmux mapping in board-specific DTS.
prima2.dtsi provides all possible (groups,functions) configuration, and
device in .dts select configurations from dtsi files.

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>