hw/misc: add a TMP42{1, 2, 3} device model

hw/misc: add a TMP42{1, 2, 3} device model

Largely inspired by the TMP105 temperature sensor, here is a model for
the TMP42{1,2,3} temperature sensors.

Specs can be found here :

	http://www.ti.com/lit/gpn/tmp421

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1496739230-32109-2-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>