ARM: vexpress: add sched_clock() for Versatile Express

Add a sched_clock() implementation to Versatile Express using the new
sched_clock() infrastructure for extending 32bit counters to full
64-bit nanoseconds.

Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>