pinctrl: imx: fix initialization of imx_pinctrl_desc

pinctrl: imx: fix initialization of imx_pinctrl_desc

To i.MX7D, there are two iomux controllers, iomuxc and iomuxc_lpsr.
They should not share one pin controller descriptor, otherwise
the value filled into imx_pinctrl_desc when probing the first
iomux controller will be overridden when probing the second one.

In this patch, discard the static allcoated imx_pinctrl_desc and
switch to dynamically allcate pin controller descriptor for each
iomux controller.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>