arm64: dts: imx8mp-venice-gw702x: fix GSC vdd_bat data size

On this board, vdd_bat is 16bit, not 24bit. Set the mode to
mode_voltage_16bit (3) instead of mode_voltage_24bit (1).

Fixes: 0d5b288c ("arm64: dts: freescale: Add imx8mp-venice-gw7905-2x")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>