ARM: dts: keystone: add gpio device entry

This patch adds Keystone GPIO IP device definitions in DT which supports
up to 32 GPIO lines and each GPIO line can be configured as separate
interrupt source (so called "unbanked" IRQ).

For more information see:
 http://www.ti.com/lit/ug/sprugv1/sprugv1.pdf



Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>