ARM: ux500: fixup magnetometer pins

GPIO31 and GPIO32 (CTS/RTS) are handled by the magnetometer and
shall not simultaneously be controlled by UART2.

Reported-by: Peter Nessrup <peter.nessrup@stericsson.com>
Signed-off-by: Jorgen Jonsson <jorgen.jonsson@stericsson.com>
Reviewed-by: Patrice Chotard <patrice.chotard@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>