dt-bindings: leds: is31fl319x: Document variants specificities

Add si-en compatibles for all chip variants and add conditionals
depending on compatibles to document variants specs:
- possible reg addresses
- whether audio-gain-db is supported or not
- maximum number of leds
- led-max-microamp values

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vincent Knecht <vincent.knecht@mailoo.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220712100841.1538395-3-vincent.knecht@mailoo.org