Add platform hook-up interface to support I2C based GPIo expander
(TCA6416).

There are 3 instances of I2C Expander on AM3517EVM,
	- One is over I2C-2 mounted on Base board
	- Two are over I2C3 mounted on UI Card
Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>