ARM: OMAP2+: Fix external clock support for dmtimers

Currently, the dmtimer determines whether an timer can support an external
clock source (sys_altclk) for driving the timer by the IP version. Only
OMAP24xx devices can support an external clock source, but the IP version
between OMAP24xx and OMAP3xxx is common and so this incorrectly indicates
that OMAP3 devices can use an external clock source.

Rather than use the IP version, just let the clock framework handle this.
If the "alt_ck" does not exist for a timer then the clock framework will fail
to find the clock and hence will return an error. By doing this we can eliminate
the "timer_ip_version" variable passed as part of the platform data and simplify
the code.

We can also remove the timer IP version from the HWMOD data because the dmtimer
driver uses the TIDR register to determine the IP version.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>