pinctrl: aspeed-g6: Add AST2600 pinconf support

The AST2600 pinconf is a little different from previous generations of
ASPEED BMC SoCs in terms of architecture. The pull-down setting is
per-pin setting now, and drive-strength support 4 kind of value (e.g.
4ma, 8ma, 12ma, 16ma).

Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
[AJ: Trim unused pinctrl register macros]
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191202061432.3996-8-andrew@aj.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>