drivers: gpio: Add DTS support for Stellaris gpio driver.

    * Add DTS support for gpio controller driver

Signed-off-by: Ashokkumar B <ashokkumar@zilogic.com>
Signed-off-by: Subash G <subash@zilogic.com>
Signed-off-by: Vishnu K <vishnu@zilogic.com>
Signed-off-by: Vaishnavi D <vaishnavi.d@zilogic.com>