sched/walt: Drop arch-specific timer access

On at least one platform, occasionally the timer providing the wallclock
was able to be reset/go backwards for at least some time after wakeup.

Accept that this might happen and warn the first time, but otherwise just
carry on.

Change-Id: Id3164477ba79049561af7f0889cbeebc199ead4e
Signed-off-by: Chris Redpath <chris.redpath@arm.com>