thermal/drivers/generic-adc: Register thermal zones as hwmon sensors

Register thermal zones as hwmon sensors to let userspace read
temperatures using standard hwmon interface.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
[Yangtao: only keep devm_thermal_add_hwmon_sysfs]
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230620090732.50025-11-frank.li@vivo.com