MLK-12252 dts: imx7d: move lpsr pin definition into a separated head file

MLK-12252 dts: imx7d: move lpsr pin definition into a separated head file

Originally we put all the definition of both imx7d normal pins and
lpsr pins in imx7d-pinfunc.h which may lead to a easy failure of
user by wrongly put the normal pins of &iomuxc under &iomuxc_lpsr
node due to user has no idea about the difference, then pinctrl
driver will take the wrong value to set which may potentially break
other devices to work.

We have met this issue several times and it's hard to debug when it
happens.

This patch separates the lpsr pins into a dedicated head file
to give user a reminder to put lpsr pins group under the correct
pinctrl device node.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
(cherry picked from commit c524454c24fd9e5e329351dd154cbd24d47d0e0e)