dt-bindings: mfd: Ensure 'syscon' has a more specific compatible

Using 'syscon' alone should be avoided as compatible strings should be
specific enough to identify the exact set of registers contained. Update
the schema to ensure at least 2 compatible strings are present.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>