pinctrl: make it possible to add multiple maps

Since we now anyway make a copy of the platform-supplied pinmux
map, we can just as well make it possible to call the function
adding maps several times, so as to simplify cases (as PXA) where
several sets of disparate mappings need to be added depending on
target platform.

Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Acked-by: Arnd Bergmann <arnd@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>