docs: counter: Include counter-chrdev kernel-doc to generic-counter.rst

The counter-chrdev.c file exports the counter_push_event() function.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/20211005061437.4846-1-vilhelm.gray@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>