One of the usage of the LRADC is to implement buttons. The bindings define
that we should have one subnode per button, with their associated voltage
as a property.

However, there was no reg property but we still used the voltage associated
to the button as the unit-address, which eventually generated warnings in
DTC.

Rename the node names to avoid those warnings.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>