pinctrl: aspeed-g6: Fix UART13 group pinmux

When UART13G1 is set the pinmux configuration in SCU4B8 for UART13G0
should be cleared.

Fixes: 58dc52ad00a0 ("pinctrl: aspeed: Add AST2600 pinmux support")
Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
[AJ: Tweak commit message]
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191008044153.12734-7-andrew@aj.id.au


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