Currently, every SoC driver defines struct pinctrl_desc statically,
i.e. it consumes memory footprint even if it is not probed.

In multi-platform, many pinctrl drivers are linked (generally as
built-in objects), although only one of them is actually used.
So, it is reasonable to allocate memory dynamically where possible.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>