ARM: dts: pandaboard es: add bluetooth uart for HCI

The wl271 bluetooth uart is connected to uart2.
Setup a serdev uart child and separate bluetooth and uart2 pinmux
from wl12xx pinmux to better group the pins and muxes.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>