taprio: Add support for setting flags

taprio: Add support for setting flags

This allows a new parameter, flags, to be passed to taprio. Currently, it
only supports enabling the txtime-assist mode. But, we plan to add
different modes for taprio (e.g. hardware offloading) and this parameter
will be useful in enabling those modes.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: David Ahern <dsahern@gmail.com>