pinctrl: single: dump pinmux register value

Dump pinmux register value, not only function part in the pinmux
register.

Also fix the issue on caluclating pin offset. The last parameter
should be pin number, not register offset.

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