dt-bindings: rtc: Drop isil,isl12057.txt

The "isil,isl12057" compatible is already supported by rtc-ds1307.yaml,
so remove the old text binding.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250807214414.4172910-1-robh@kernel.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>