gpio: mxc: add power management support

GPIO registers could lose context on i.MX7D, when
enter LPSR mode, the whole SoC will be powered off
except LPSR domain, GPIO banks will lose context
in this case, need to restore the context after
resume from LPSR mode.

This patch adds new compatible string for i.MX7D
which supports GPIO power off feature in suspend,
and adds the GPIO save/restore operations in noirq
suspend/resume phase, since GPIO is fundamental
module which could be used by other peripherals'
resume phase.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>