MLK-17491-13 dts: imx7ulp: improve the gpio using

MLK-17491-13 dts: imx7ulp: improve the gpio using

Currently people have no idea on which pad is correspding to which gpio
controller as there's no hints in dts. Let's add a proper prefix for gpio
nodes as follows in dts to make it much easier to use.
gpio0 = &gpio_ptc;
gpio1 = &gpio_ptd;
gpio2 = &gpio_pte;
gpio3 = &gpio_ptf;

Reviewed-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>