ARM: OMAP: dmtimer: low-power mode support

Clock is enabled only when timer is started and disabled when the the timer
is stopped. Therefore before accessing registers in functions clock is enabled
and then disabled back at the end of access. Context save is done dynamically
whenever the registers are modified. Context restore is called when context is
lost.

Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
[tony@atomide.com: updated to use revision instead of tidr]
Signed-off-by: Tony Lindgren <tony@atomide.com>