dt-bindings: efm32: remove bindings for deleted platform

Commit cc611137 ("ARM: drop efm32 platform") removed the platform,
so no need to still carry the bindings.

Signed-off-by: Wolfram Sang <wsa@kernel.org>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220615210720.6363-1-wsa@kernel.org