pinctrl: mediatek: Fix multiple registration issue.

Since our common driver need support main chip and PMU
at the same time, that means it will register two
pinctrl device, and the pinctrl_desc structure should
be used two times.

But pinctrl_desc use global static definition, then
the latest registered pinctrl device will overwrite
the old one's, all members in pinctrl_desc will set to
the new one's, such as name, pins and pins numbers, etc.
This is a bug.

Move pinctrl_desc into mtk_pinctrl, assign new value for
each pinctrl device to fix it.

Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>