arm64: dts: imx8mm-venice: Fix GSC vdd_bat data size.

On these boards, vdd_bat is 16bit, not 24bit.  Reading them as 24bit
values yield garbage values because of the additional byte, which is a
configurable fan trippoint[1].

So set their mode to mode_voltage_16bit = 3 instead of
mode_voltage_24bit = 1.

[1]: http://trac.gateworks.com/wiki/gsc#SystemTemperatureandVoltageMonitor



Only tested on GW7100.

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>