gpio: aspeed-sgpio: Rename and add Kconfig/Makefile

This renames the "gpio-aspeed" driver to conform with other
GPIO drivers as "gpio-aspeed-sgpio.c". All GPIO drivers
should start with the string "gpio-" no special exceptions.

Also the Kconfig and Makefile entries should normally
go with the driver but I missed this in my review, sorry
for mistake. "CONFIG_GPIO_ASPEED_SGPIO" is used to
activate this driver.

Cc: Hongwei Zhang <hongweiz@ami.com>
Fixes: 7db47faa

 ("gpio: aspeed: Add SGPIO driver")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190927114833.12551-1-linus.walleij@linaro.org