ARM: dts: lpc4350-hitex-eval: fix unit name warnings from dtc

Fix the following warnings from dtc by either adding or removing
the unit name from the node.

Warning (unit_address_vs_reg): Node /soc/flash-controller@40003000/flash@0 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@0 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@1 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@2 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@3 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@4 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@5 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@6 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /pca_buttons/button@7 has a unit name, but no reg property

Signed-off-by: Joachim Eastwood <manabian@gmail.com>