User specifies the clock selection by using -w CLOCK or --clock=CLOCK
where CLOCK=[REALTIME, TAI, BOOTTIME, MONOTONIC].

The default CLOCK selection is MONOTONIC.

The implementation of clock selection parsing is borrowed from
iproute2/tc/q_taprio.c
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211230035447.523177-4-boon.leong.ong@intel.com