hw/misc/bcm2835_thermal: Add a dummy BCM2835 thermal sensor

hw/misc/bcm2835_thermal: Add a dummy BCM2835 thermal sensor

We will soon implement the SYS_timer. This timer is used by Linux
in the thermal subsystem, so once available, the subsystem will be
enabled and poll the temperature sensors. We need to provide the
minimum required to keep Linux booting.

Add a dummy thermal sensor returning ~25°C based on:
https://github.com/raspberrypi/linux/blob/rpi-5.3.y/drivers/thermal/broadcom/bcm2835_thermal.c

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20191019234715.25750-2-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>