Add clocksource driver for OMAP.

This is an update of

http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=3876/1

from Daniel Walker and Kevin Hilman.

Changes from Dirk Behme:

- Apply cleanly to recent git (pt_regs change)
- Move clocksource init to extra function. Can be called
later by clocksource subsystem
- Remove warning
arch/arm/plat-omap/timer32k.c:216: warning:
'omap_32k_timer_handler' defined but not used
if CONFIG_NO_IDLE_HZ isn't set.
- Minor cleanups

Changes from David Brownell:

   - OMAP1 MPU timer:
       * consult clock framework to determine xtal clock
       * use timer1 as a real free-running clock
   - OMAP2 GP timer
       * use timer2 as a real free-running clock
   - Both: minor cleanups
   - Kconfig updates

Signed-off-by: Dirk Behme <dirk.behme_at_gmail.com>