m68knommu: MCF5307 PIT GENERIC_CLOCKEVENTS support

The PIT code has been changed in order to suppport GENERIC_CLOCKEVENTS.
The priority of the PIT clocksource has been decreased in favor of the
DMA timer.

pit_cycles_per_jiffy become a constant (PIT_CYCLES_PER_JIFFY) because it
is known at compile time and does not change afterwards.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Signed-off-by: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>