pinctrl: aspeed: Use masks to describe pinconf bitfields

Since some of the AST2600 pinconf setting are not just single bit, modified
aspeed_pin_config @bit to @mask and add @mask to aspeed_pin_config_map to
support configuring multiple bits.

Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
[AJ: Tweak commit message]
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191202061432.3996-7-andrew@aj.id.au


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