Documentation: ABI: sysfs-bus-counter: add frequency & num_overflows items

This commit adds frequency and num_overflows items to counter ABI file
(e.g. for TI ECAP hardware used in capture operating mode).

Signed-off-by: Julien Panis <jpanis@baylibre.com>
Link: https://lore.kernel.org/r/20220923142437.271328-3-jpanis@baylibre.com/
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/467ae80e97c586c6bc9c453c6156ffcb5d4853d6.1664318353.git.william.gray@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>