dt-bindings: pinctrl: aspeed: Document AST2600 pinmux

The AST260 differs from the 2400 and 2500 in that it supports multiple
groups for a subset of functions.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190729055604.13239-2-andrew@aj.id.au
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>