[ARM] orion: make sure sched_clock() usage of cnt32_to_63() is safe

With a TCLK = 200MHz, the half period of the hardware timer is roughly
10 seconds. Because cnt32_to_63() must be called at least once per
half period of the base hardware counter, it is a bit risky to rely
solely on scheduling to generate frequent enough calls. Let's use a
kernel timer to ensure this.

Signed-off-by: Nicolas Pitre <nico@marvell.com>