ARM: OMAP: Add a timer attribute for timers that can interrupt the DSP

Some instances of the DMTIMER peripheral on OMAP devices have the ability
to interrupt the on-chip DSP in addition to the ARM CPU. Add a DMTIMER
attribute to indicate which timers can interrupt the DSP. By using the
omap_dm_timer_request_by_cap() API, driver will now be able to allocate
a DMTIMER that can interrupt the DSP based upon this attribute and not require
the driver to know which instance has this capability.

DMTIMERs that have the ability to interrupt the DSP on OMAP devices are as
follows ...

- OMAP1 (OMAP5912/16xx/17xx) devices	- All 8 DMTIMERs
- OMAP2/3/4 devices			- DMTIMERs 5-8

Please note that for OMAP3+, timer8 has the ability to interrupt the DSP and
generate a PWM output.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>