pinctrl: aspeed-g6: Fix I3C3/I3C4 pinmux configuration

The documentation to configure I3C3/FSI1 and I3C4/FSI2 was initially
unclear.

Fixes: 58dc52ad00a0 ("pinctrl: aspeed: Add AST2600 pinmux support")
Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
[AJ: Tweak commit message, resolve rebase conflicts]
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191008044153.12734-5-andrew@aj.id.au


Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>