dt-bindings: pinctrl: rt2880: add binding document

The commit adds rt2880 compatible node in binding document.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20201208075523.7060-2-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>