dt-bindings: rtc: Update for new chip in isl1208 series

Since this documents multiple chips controlled by the isl1208 driver,
name it isil,isl1208.txt, as the convention is to use the base driver
name in the bindings document for drivers with multiple devices.

Include all chips supported by this driver in the docs.  Make it
clear which properties apply to which chips.

Expand documentation for existing isil,evienb property to explain
operation in more detail.  Existing docs just describe it in terms of
setting a bit in a register.

CC: Alessandro Zummo <a.zummo@towertech.it>
CC: Alexandre Belloni <alexandre.belloni@bootlin.com>
CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: devicetree@vger.kernel.org
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>