pinctrl: aspeed-g6: Add AST2600 I3C1 and I3C2 pinmux config

These pins only expose a single function but are not fixed-function as
their I3C capability can be disabled.

Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
[AJ: Tweak commit message, sort pins list]
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191202061432.3996-3-andrew@aj.id.au


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>