da850-evm: add baseboard GPIO expander buttons, switches and LEDs

This patch adds a pca953x platform device for the tca6416 found on the evm
baseboard. The tca6416 is a GPIO expander, also found on the UI board at a
separate I2C address. The pins of the baseboard IO expander are connected to
software reset, deep sleep enable, test points, a push button, DIP switches and
LEDs.

Add support for the push button, DIP switches and LEDs and test points (as
free GPIOs). The reset and deep sleep enable connections are reserved by the
setup routine so that userspace can't toggle those lines.

The existing tca6416-keypad driver was not employed because there was no
apararent way to register the LEDs connected to gpio's on the tca6416 while
simultaneously registering the tca6416-keypad instance.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Reviewed-by: Chris Cordahi <christophercordahi@nanometrics.ca>
CC: Govindarajan, Sriramakrishnan <srk@ti.com>
Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
CC: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>