rockchip: adc: rockchip-saradc: use union for preparing for v2

The registers are entirely different between SARADC v1 and SARADC v2, so
let's prepare to add another struct for accessing v2 registers by adding
a union.

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