pinctrl: aspeed: g6: Fix PWMG0 pinctrl setting

The SCU offset for signal PWM8 in group PWM8G0 is wrong, fix it from
SCU414 to SCU4B4.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Fixes: 2eda1cde

 ("pinctrl: aspeed: Add AST2600 pinmux support")
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20201217024912.3198-1-billy_tsai@aspeedtech.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>