[ARM] orion: sched_clock implementation for orion platforms

sched_clock implementation for orion platform. Its realized using
free-running clocksource timer, which provides a resolution of 7.5ns
(depending on tclk). It's derived from PXA's sched_clock implementation.

[ nico: renamed orion2ns to tclk2ns, fixed max value in the comment ]

Signed-off-by: Stefan Agner <stefan.agner@yahoo.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>