ARM: ux500: move AB8500 EXTCPENA from board file to DT

This moves the configuration of the AB8500 EXTCPENA pin
from the board file to the device tree.

Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>