ARM: dts: lpc32xx: ea3250: beautify gpio keys children nodes

Regarding the 'gpio_keys' device node a dtc reports a couple of
warnings:

  Warning (avoid_unnecessary_addr_size): /gpio_keys: unnecessary
  #address-cells/#size-cells without "ranges" or child "reg" property

  Warning (unit_address_vs_reg): /gpio_keys/button@21: node has
  a unit name, but no reg property

The change fixes these issues and adds empty lines between adjacent
children device nodes. The device node itself is renamed by substituting
an underscore by hyphen to follow the standard naming convention
of device tree nodes.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>