gpio: aspeed: port Linux dt-bindings header file

Makes it easier to add readable GPIO definitions in DTS files
for Aspeed SOC based boards.

Ported with small edits to add IBM copyright statement and fix
for checkpatch warning.

Signed-off-by: Dhananjay Phadke <dphadke@linux.microsoft.com>
Reviewed-by: Billy Tsai <billy_tsai@aspeedtech.com>
Acked-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>