gpio: gpio-aspeed-sgpio: Return error if ngpios is not multiple of 8.

Add an else-if condition in the probe function to check whether ngpios is
multiple of 8.
Per AST datasheet, numbers of available serial GPIO pins in Serial GPIO
Configuration Register must be n bytes. For instance, if n = 1, it means
AST SoC supports 8 GPIO pins.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>