rockchip: boot_mode: fix rockchip_dnl_key_pressed requiring ADC support

ADC support is implied by the Rockchip arch Kconfig but that means it
should be possible to disable ADC support and still be able to build.

However the weak implementation of rockchip_dnl_key_pressed() currently
blindly use functions from the ADC subsystem which do not exist when ADC
is not enabled, failing the build.

Therefore, let's encapsulate this logic with a check on the ADC symbol
being selected.

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>