ARM: dts: sun4i: Remove gpio-keys warnings

Some gpio-keys definitions in our DTs were having buttons defined with a
unit-address and that would generate a DTC warning.

Change the buttons node names to remove the warnings.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>