ARM: dts: imx6ull: update vdd_soc voltage for 900MHz operating point

Update VDD_SOC voltage to 1.25V for 900MHz operating point
according to datasheet Rev. 1.3, 08/2018, 25mV is added to
the minimum allowed values to cover power supply ripple.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>