Add touchscreen support for AM437x GP EVM using pixcir
touchscreen controller.

CC: Benoit Cousson <bcousson@baylibre.com>
CC: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>