rockchip: imply ADC and SARADC_ROCKCHIP for Rockchip SOCs

Enable the Rockchip SARADC driver for all Rockchip SoCs.

Note that the SARADC peripheral is available on all SoCs except the
RK3036 and RK3228.  However, as this is a DM-driver, enabling by
default will not cause any function problems (and can always be
changed from defconfig, if size is a concern).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: David Wu <david.wu@rock-chips.com>