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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>