ARM: dts: imx: ventana: add alternate PWM4 pinmux for GW54xx

The GW54xx can provide PWM4 out either the off-board backlight connector
or the off-board digital I/O connector. By default the pinmux routes it
to the backlight connector but this pinctl alternate provides documentation
for those who may want to change it.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>