thermal: amlogic: Add hwmon support

Many monitoring tools read the CPU temperature using the hwmon
interface. Expose the thermal sensors on Amlogic boards as hwmon
devices.

Without this lm_sensors' "sensors" tool does not find any temperature
sensors. Now it prints:
  cpu_thermal-virtual-0
  Adapter: Virtual device
  temp1:        +44.7 C  (crit = +110.0 C)

  ddr_thermal-virtual-0
  Adapter: Virtual device
  temp1:        +45.9 C  (crit = +110.0 C)

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201115190658.631578-1-martin.blumenstingl@googlemail.com