ARM: OMAP: Add dmtimer interrupt disable function

The OMAP dmtimer driver does not currently have a function to disable the
timer interrupts. For some timer instances the timer interrupt enable
function can be used to disable the interrupts because the same interrupt
enable register is used to disable interrupts. However, some timer instances
have separate interrupt enable/disable registers and so this will not work.
Therefore, add a dedicated function to disable interrupts.

This change is required for OMAP4+ devices. For OMAP4, all timers apart from 1,
2 and 10 need this function and for OMAP5 all timers need this function.
Please note that the interrupt disable function has been written so that it
can be used by all OMAP devices.

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