iio: adc: rockchip_saradc: add support for rk3066-tsadc variant

Older Rockchip SoCs, at least the rk3066, used a slightly modified saradc
for temperature measurements. This so called tsadc does not contain any
active parts like temperature interrupts and only supports polling the
current temperature. The returned voltage can then be converted by a
suitable thermal driver to and actual temperature and used for thermal
handling.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>