ARM: dts: imx7d-sdb: Add GPIO expander node

Add node for U38, a 74LV595PW serial-in shift register that acts as a
GPIO expander on the board.

Cc: yurovsky@gmail.com
Cc: Dong Aisheng <aisheng.dong@nxp.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>