configs: arm64/bcm2711: Use CONFIG_BRCMSTB_THERMAL instead of CONFIG_BCM2835_THERMAL

The Raspberry Pi 4 uses the brcmstb thermal driver rather than brcm2835,
based on the device tree compatible string 'brcm,avs-tmon-bcm2838'. With
CONFIG_BRCMSTB_THERMAL enabled, reading temperature from
/sys/class/thermal/thermal_zone0/temp works as expected instead of
returning EINVAL.

Fixes: https://github.com/raspberrypi/linux/issues/3071


Signed-off-by: Allen Wild <allenwild93@gmail.com>