pinctrl: aspeed-g6: Add bias controls for 1.8V GPIO banks

These were skipped in the original patches adding pinconf support for
the AST2600.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Cc: Johnny Huang <johnny_huang@aspeedtech.com>
Link: https://lore.kernel.org/r/20200910025631.2996342-4-andrew@aj.id.au


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