Documentation: ABI: sysfs-bus-counter: add cascade_counts_enable and external_input_phase_clock_select

This commit adds cascade_counts_enable and external_input_phase_
clock_select items to counter ABI file.
(e.g. for Renesas MTU3 hardware used for phase counting).
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230330111632.169434-4-biju.das.jz@bp.renesas.com