pinctrl: aspeed-g6: Rename SD3 to EMMC and rework pin groups

AST2600 EMMC support 3 types DAT bus sizes (1, 4 and 8-bit),
corresponding to 3 groups: EMMCG1, EMMCG4 and EMMCG8

Fixes: 58dc52ad00a0 ("pinctrl: aspeed: Add AST2600 pinmux support")
Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191008044153.12734-8-andrew@aj.id.au


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