pinctrl: pinctrl-aspeed-g6: Add sgpio pinctrl settings

AST2600 supports 2 SGPIO master interfaces and 2 SGPIO slave interfaces.
Current pinctrl driver only define the first sgpio master and slave
interfaces.
The second SGPIO master and slave interfaces should be added in
pinctrl driver as well.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210525055308.31069-4-steven_lee@aspeedtech.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>