bcm2708: use a constant SC_MULT / SC_SHIFT

The calculated values somehow don't agree with the sched_clock code and
we end up with warnings like:

sched_clock: wrong multiply/shift: 2097152000>>21 vs calculated 4194304000>>22
sched_clock: fix multiply/shift to avoid scheduler hiccups

So use the constant values much like arch/arm/mach-tegra/timer.c does.

Signed-off-by: Chris Boot <bootc@bootc.net>