armv8: fsl-layerscape: Add support of GPIO structure

Layerscape Gen2 SoC supports GPIO registers to control GPIO
signals. Adding support of GPIO structure to access GPIO
registers.

Signed-off-by: Pratiyush Srivastava <pratiyush.srivastava@nxp.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Signed-off-by: Calvin Johnson <calvin.johnson@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>