rockchip: boot_mode: find the saradc device name

adc_channel_single_shot() requires the full device name e.g.
"saradc@ff100000", which differs between Rockchip SoC's, but they all
share the prefix "saradc"; find the ADC device with this name prefix and
use its full name.

Signed-off-by: Hugh Cole-Baker <sigmaris@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>