ARM: dts: imx6qdl-rex: add gpio expander pca9535

The pca9535 gpio expander is present on the Rex baseboard, but missing
from the dtsi.
The pca9535 is on i2c2 bus which is common to the three SOM
variants (Basic/Pro/Ultra), thus it is activated by default.

Add also the new gpio controller and the associated interrupt line
MX6QDL_PAD_NANDF_CS3__GPIO6_IO16.

Signed-off-by: Gilles DOFFE <gilles.doffe@savoirfairelinux.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>