ARM: mxc: Add sched_clock to mxc platform

Add sched_clock using cyc_to_sched_clock and update_sched_clock
with HAVE_SCHED_CLOCK
tested on iMX27 and iMX35

Signed-off-by: Jan Weitzel <j.weitzel@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>