arm: Stub out NRF51 TWI magnetometer/accelerometer detection

Recent microbit firmwares panic if the TWI magnetometer/accelerometer
devices are not detected during startup.  We don't implement TWI (I2C)
so let's stub out these devices just to let the firmware boot.

Signed-off by: Steffen Görtz <contrib@steffen-goertz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190110094020.18354-2-stefanha@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: fixed comment style]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>