boards: dts: Fix dtc warning in board dts files

This patch is equivalent to 7b0ce852,
but applied to buttons. As stated in the previous commit message:

This change aims at fixing 'unit_address_vs_reg' warning in arm based
boards. This warning pops up when a node name is made up with an
address (node_name@xx) but does not contain a reg property. This case
was encountered for led nodes for instance, where a reg property has
no meaning.  Fix this by changing node_name@xx to node_name_xx which
removes the guilty '@XX' syntax but preserves node_name uniqueness.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>