dt-bindings: i2c: stm32-i2c: Move st,syscfg-fmp definition to top level

It is preferred to define all properties in the main schema and leave
if/then/else schemas to just be further constraints on properties.

Rework the schema to use be more specific for each cell. Previously,
multiple entries of 3 cells each was allowed.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220119174407.3810088-1-robh@kernel.org