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 kernel (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>