iop: implement sched_clock()

This adds a better sched_clock() to the IOP platform,
implemented using its new clocksource support.

Tested on n2100, compile-tested for all plat-iop machines.

[dan.j.williams@intel.com: allow early cp6 access]
Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>