dt-bindings: pinctrl: aspeed: Split bindings document in two

Have one for each of the AST2400 and AST2500. The only thing that was
common was the fact that both support ASPEED BMC SoCs.

Cc: Johnny Huang <johnny_huang@aspeedtech.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20190628023838.15426-2-andrew@aj.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>