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>