configs: Enable BCM2835 thermal driver in kernel8

The arm64 version of bcm2711_defconfig is intended for Pi 4, but the
Raspberry Pi OS kernel8.img built from it is also used for Pi 3. It is
therefore necessary to include some legacy drivers such as the BCM2835
thermal driver, otherwise there is no support for reading the CPU
temperature on a Pi 3 in 64-bit mode.

See: https://github.com/raspberrypi/linux/issues/4077



Signed-off-by: Phil Elwell <phil@raspberrypi.com>