LF-6142 arm64: dts: freescale: Correct gpio power key

On i.MX8ULP 9*9 EVK baord, the GPIO used for SW4 is changed
to port PTF26, so update the gpio key property to correct it.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>