LF-4511 arm64: dts: freescale: Correct the cpu supply on imx8mn ddr3l evk board

For i.MX8MN DDR3L EVK board, it uses a 11x11 package that VDD_ARM & VDD_SOC
is combined together, the 'cpu-supply' should be buck1, so correct it.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Acked-by: Jason Liu <jason.hui.liu@nxp.com>