pinctrl: aspeed: Document existence of deprecated compatibles

Otherwise they look odd in the face of not being listed in the bindings
documents.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190724081313.12934-3-andrew@aj.id.au


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