docs: Add Generic Counter interface documentation

This patch adds high-level documentation about the Generic Counter
interface.
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/index.rst

@@ -56,6 +56,7 @@ available subsections can be seen below.
    slimbus
    soundwire/index
    fpga/index
+   generic-counter
 
 .. only::  subproject and html
 

MAINTAINERS

@@ -4059,6 +4059,7 @@ M:	William Breathitt Gray <vilhelm.gray@gmail.com>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-bus-counter*
+F:	Documentation/driver-api/generic-counter.rst
 F:	drivers/counter/
 F:	include/linux/counter.h
 F:	include/linux/counter_enum.h