arm64: dts: imx8mm-venice-gw7901: add digital I/O direction control GPIO's

The GW7901 has GPIO's to configure the direction of its isolated
digital I/O signals. Add the GPIO pinmux and line names.

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