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.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/driver-api/generic-counter.rst

@@ -237,6 +237,9 @@ for defining a counter device.
 .. kernel-doc:: drivers/counter/counter-core.c
    :export:
 
+.. kernel-doc:: drivers/counter/counter-chrdev.c
+   :export:
+
 Driver Implementation
 =====================