ethernet: ti: am65-cpts: add routines to support taprio offload

TAPRIO/EST offload support in CPSW2G requires EST scheduler
function enabled in CPTS. So this patch add a function to
set cycle time for EST scheduler.  It also add a function for
getting time in ns of PHC clock for taprio qdisc configuration.
Mostly to verify if timer update is needed or to get actual
state of oper/admin schedule.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>