ARM: ux500: move AB8500 modem I2C settings to DT

This moves the pin setup of the AB8500 modem I2C pins
(SCL/SDA) 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>