ARM: arch_timer: use full 64-bit counter for sched_clock

Only 32-bits of the arch timer were being used and wrapping was needlessly
being done in s/w. By using the full counter (56-64 bits), we don't need
to deal with wrapping and can simplify the implementation when using
arch timer.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>