pinctrl: sirf: add missing DT-binding document

While sending email to Linus for reviewing:
"pinctrl: sirf: add DT-binding pinmux mapping support"
https://patchwork.kernel.org/patch/1364361/

i have included the devicetree/bindings/pinctrl/pinctrl-sirf.txt

But while sending pull request with commit 056876f6c73406c,
i missed the document.

this patch takes the document back.

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