boards/arm: Fix button dts syntax for stm32 based boards

This change aims at fixing 'unit_address_vs_reg' warning in
STM32 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 button 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: Erwan Gouriou <erwan.gouriou@linaro.org>