gpio: aspeed: Add SGPIO driver

Add SGPIO driver support for Aspeed AST2500 SoC.
Signed-off-by: Hongwei Zhang <hongweiz@ami.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/1566335128-31498-2-git-send-email-hongweiz@ami.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>