dt-bindings: input: mediatek,pmic-keys: Drop incomplete example

dt-bindings: input: mediatek,pmic-keys: Drop incomplete example

The example for the Mediatek PMIC keys is incomplete as the binding is
the full PMIC, not just the sub-functions. It is preferred for MFD
examples to be complete in the top-level MFD device binding rather than
piecemeal in each sub-function binding.

This also fixes an undocumented (by schema) compatible warning for
"mediatek,mt6397".

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231128214816.3975893-1-robh@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>