pinctrl: aspeed: Add PIN_DECL_3() helper

This case is common in the AST2600, so add to the collection.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190729055604.13239-4-andrew@aj.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>