Input: add keypad driver for keys interfaced to TCA6416

This patch implements a simple Keypad driver that functions
as an I2C client. It handles key press events for keys
connected to TCA6416 I2C based IO expander.

Signed-off-by: Sriramakrishnan <srk@ti.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>