MAINTAINERS: Add Counter subsystem git tree

The Counter subsystem tree is now separate from the IIO tree.
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/41db1ea7bd9384da99fb332019ddc03067fe5311.1647373009.git.vilhelm.gray@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -5017,6 +5017,7 @@ COUNTER SUBSYSTEM
 M:	William Breathitt Gray <vilhelm.gray@gmail.com>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
+T:	git git@gitlab.com:vilhelmgray/counter.git
 F:	Documentation/ABI/testing/sysfs-bus-counter
 F:	Documentation/driver-api/generic-counter.rst
 F:	drivers/counter/