ARM: OMAP: Add clocksource driver for OMAP

ARM: OMAP: 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: 'omap_32k_timer_handler' defined but not used
     when CONFIG_NO_IDLE_HZ isn't set.
   - Minor cleanups

Changes from David Brownell:

   - OMAP1, OMAP2: 32k clocksource is always available on 16xx and 24xx
   - OMAP1 MPU timer:
       * consult clock framework to determine xtal clock
       * bugfix sched_clock() low order bits
       * cleanup
   - Kconfig updates

Note that "MPU timer" still doesn't have lost tick support.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dirk Behme <dirk.behme_at_gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>