ARM: OMAP: Don't restore of DMTIMER TISTAT register

The timer TISTAT register is a read-only register and therefore restoring the
context is not needed. Furthermore, the context of TISTAT is never saved
anywhere in the current code. The TISTAT register is read-only for all OMAP
devices from OMAP1 to OMAP4. OMAP5 timers no longer have this register.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>