rockchip: adc: rockchip-saradc: factor out channel_data callback

SARADC v1 and v2 have a different way of reading data, therefore let's
abstract this function so that it can be provided from the udevice.data
pointer.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>