[IPROUTE]: Introduce TIME_UNITS_PER_SEC to represent internal clock resolution

Introduce TIME_UNITS_PER_SEC and conversion functions between internal
resolution and resolution expected by the kernel (currently implemented as
NOPs, only needed by HFSC, which currently always uses microseconds).
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>