arm64: dts: imx8mn: Adjust 1.2GHz OPP voltage to OD mode

arm64: dts: imx8mn: Adjust 1.2GHz OPP voltage to OD mode

According to latest datasheet Rev.0, 10/2019, there is restriction
as below:

"If VDD_SOC/GPU/DDR = 0.95V, then VDD_ARM must be >= 0.95V."

As by default SoC is running at OD mode(VDD_SOC = 0.95V), so
VDD_ARM 1.2GHz OPP's voltage should be increased to 0.95V.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>