pinctrl: ingenic: Add lcd-8bit group for JZ4770

Add the "lcd-8bit" group to the "lcd" function.

As "lcd-24bit" is a superset of "lcd-8bit", in theory the former could
be modified to only contain the pins not already included in "lcd-8bit",
just like how it's done for the JZ4740 and JZ4725B platforms. However,
we can't do that without breaking Device Tree ABI, so in that case we
have no choice but to have two groups containing the same pins.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@wanyeetech.com>
Link: https://lore.kernel.org/r/20201101090104.5088-3-paul@crapouillou.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>