gpio: da8xx_gpio: Add "ti,keystone-gpio" compatible

Add "ti,keystone-gpio" compatible so as be able to use Linux DT files as
is.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>