dt-bindings: gpio: aspeed: Add SGPIO support

Add bindings to support SGPIO on AST2400 or AST2500.

Signed-off-by: Hongwei Zhang <hongweiz@ami.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/1564603297-1391-2-git-send-email-hongweiz@ami.com


[Adjusted when applying in several ways]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>