dt-bindings: rtc: st,stm32-rtc: Make each example a separate entry

Each independent example should be a separate entry. This allows for
'interrupts' to have different cell sizes.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220106182518.1435497-7-robh@kernel.org